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

tsconfig.json

@@ -6,6 +6,9 @@
     "moduleResolution": "node",
     "allowJs": true,
     "strict": true,
+    "declaration": true,
+    "declarationMap": true,
+    "declarationDir": "./dist/types",
     "noImplicitAny": false,
     "allowSyntheticDefaultImports": true,
     "experimentalDecorators": true,
@@ -34,5 +37,5 @@
     ],
     "skipLibCheck": true
   },
-  "include": ["src/**/*.ts", "src/**/*.js"]
+  "include": ["src/**/*.ts", "src/**/*.js", "src/**/*.d.ts"]
 }