Update NPM packaging, linting, and a bunch of types (#275)

Preparation for
https://github.com/webrecorder/archiveweb.page/issues/198 /
https://github.com/webrecorder/archiveweb.page/pull/199.

## Changes

- Organizes published files more cleanly, using `dist/types` folder as
the target for type declarations and sourcemaps
- Configures type field in `package.json`
- Updates ESLint & its config
  - Sets up type-informed linting rules mirroring browsertrix
- Adds a bunch of other useful eslint and eslint-typescript rules, and
applies auto-fixes to the codebase
- Cleans up a bunch of types throughout the codebase, particularly parts
of the code used by ArchiveWebPage
- Changes `ItemIndex#_deleting` from `private` to `protected` so that a
part of AWP that extends it can do so properly
- Makes a bunch of previously-implied [non-null
assertions](https://www.typescriptlang.org/docs/handbook/2/everyday-types.html#non-null-assertion-operator-postfix-)
explicit
- Updates `URLResource` and `ItemType` types (and renames `Item` →
`ItemType` to disambiguate from the `Item` class)
- Removes `node-sass` and replaces it with `sass-embedded` for much
quicker installs and compilation

.eslintrc.js

@@ -1,32 +1,110 @@
 /* eslint-env node */
-/** @type { import("eslint").Linter.Config } */
+/** @type {import('eslint').Linter.Config} */
 module.exports = {
+  parser: "@typescript-eslint/parser",
   env: {
     browser: true,
-    es6: true,
+    commonjs: true,
+    es2017: true,
   },
   extends: [
     "eslint:recommended",
+    "plugin:@typescript-eslint/eslint-recommended",
+    "plugin:wc/recommended",
     "plugin:lit/recommended",
     "prettier",
-    "plugin:@typescript-eslint/recommended",
   ],
   plugins: ["@typescript-eslint", "lit"],
-  parser: "@typescript-eslint/parser",
   parserOptions: {
-    ecmaVersion: 2018,
-    sourceType: "module",
+    project: ["./tsconfig.eslint.json"],
+    tsconfigRootDir: __dirname,
   },
+  root: true,
   rules: {
+    /* start stylistic rules */
+    "@typescript-eslint/adjacent-overload-signatures": "error",
+    "@typescript-eslint/array-type": "error",
+    "@typescript-eslint/consistent-type-imports": [
+      "error",
+      {
+        fixStyle: "inline-type-imports",
+      },
+    ],
+    "@typescript-eslint/prefer-readonly": "warn",
+    "@typescript-eslint/class-literal-property-style": ["warn", "getters"],
+    "@typescript-eslint/consistent-generic-constructors": "error",
+    "@typescript-eslint/consistent-type-assertions": "error",
+    "@typescript-eslint/no-confusing-non-null-assertion": "warn",
+    "@typescript-eslint/no-inferrable-types": "warn",
+    "@typescript-eslint/non-nullable-type-assertion-style": "warn",
+    "@typescript-eslint/prefer-for-of": "warn",
+    // "@typescript-eslint/prefer-nullish-coalescing": "warn",
+    "@typescript-eslint/prefer-optional-chain": "warn",
+    "@typescript-eslint/prefer-string-starts-ends-with": "error",
+    /* end stylistic rules */
+
+    /* start recommended rules */
     "no-restricted-globals": [2, "event", "error"],
-    "linebreak-style": ["error", "unix"],
+    "@typescript-eslint/no-base-to-string": "warn",
+    "@typescript-eslint/no-duplicate-enum-values": "error",
+    "@typescript-eslint/no-duplicate-type-constituents": "warn",
+    "@typescript-eslint/no-explicit-any": "warn",
+    "@typescript-eslint/no-extra-non-null-assertion": "error",
+    // "@typescript-eslint/no-floating-promises": "warn",
+    "@typescript-eslint/no-for-in-array": "warn",
+    "no-unused-vars": "off",
+    "@typescript-eslint/no-unused-vars": [
+      "warn",
+      {
+        argsIgnorePattern: "^_",
+        varsIgnorePattern: "^_",
+        destructuredArrayIgnorePattern: "^_",
+      },
+    ],
+    "no-implied-eval": "off",
+    "@typescript-eslint/no-implied-eval": "error",
+    "no-loss-of-precision": "off",
+    "@typescript-eslint/no-loss-of-precision": "warn",
+    "@typescript-eslint/no-misused-new": "error",
+    "@typescript-eslint/no-misused-promises": [
+      "error",
+      { checksVoidReturn: { arguments: false } },
+    ],
+    "@typescript-eslint/no-non-null-asserted-nullish-coalescing": "error",
+    "@typescript-eslint/no-non-null-asserted-optional-chain": "warn",
+    "@typescript-eslint/no-redundant-type-constituents": "warn",
+    "@typescript-eslint/no-this-alias": "warn",
+    "@typescript-eslint/no-unnecessary-type-assertion": "warn",
+    "@typescript-eslint/no-unnecessary-type-constraint": "warn",
+    // "@typescript-eslint/no-unsafe-argument": "warn",
+    // "@typescript-eslint/no-unsafe-assignment": "warn",
+    // "@typescript-eslint/no-unsafe-call": "warn",
+    "@typescript-eslint/no-unsafe-declaration-merging": "warn",
+    "@typescript-eslint/no-unsafe-enum-comparison": "warn",
+    // "@typescript-eslint/no-unsafe-member-access": "warn",
+    // "@typescript-eslint/no-unsafe-return": "warn",
+    "@typescript-eslint/prefer-as-const": "warn",
+    "require-await": "off",
+    "@typescript-eslint/require-await": "warn",
+    "@typescript-eslint/restrict-template-expressions": "warn",
+    "@typescript-eslint/unbound-method": "off",
   },
-  ignorePatterns: [
-    "ruffle/**/*",
-    "build/**/*",
-    "/sw.js",
-    "/ui.js",
-    "dist/**/*",
-  ],
   reportUnusedDisableDirectives: true,
+  ignorePatterns: [
+    "__generated__",
+    "__mocks__",
+    "dist",
+    "ruffle",
+    "ui.js",
+    "sw.js",
+  ],
+  overrides: [
+    {
+      extends: ["plugin:@typescript-eslint/disable-type-checked"],
+      files: ["webpack.*.js"],
+      rules: {
+        "@typescript-eslint/no-var-requires": "off",
+      },
+    },
+  ],
 };