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

src/index.ts

@@ -0,0 +1,46 @@
+import { ReplayWebApp } from "./appmain";
+import { Chooser } from "./chooser";
+import { ItemIndex } from "./item-index";
+import { ItemInfo } from "./item-info";
+import { Item } from "./item";
+import { Story } from "./story";
+import { GDrive } from "./gdrive";
+import { Loader } from "./loader";
+import { Pages } from "./pages";
+import { PageEntry } from "./pageentry";
+import { Replay } from "./replay";
+import { Sorter } from "./sorter";
+import { URLResources } from "./url-resources";
+import { Embed } from "./embed";
+import "./shoelace";
+
+export {
+  ReplayWebApp,
+  Chooser,
+  ItemIndex,
+  ItemInfo,
+  Item,
+  /**
+   * @deprecated use {@linkcode ItemIndex}
+   */
+  ItemIndex as CollIndex,
+  /**
+   * @deprecated use {@linkcode ItemInfo}
+   */
+  ItemInfo as CollInfo,
+  /**
+   * @deprecated use {@linkcode Item}
+   */
+  Item as Coll,
+  Story,
+  GDrive,
+  Loader,
+  Pages,
+  PageEntry,
+  Replay,
+  Sorter,
+  URLResources,
+  Embed,
+};
+
+export type { ItemType, URLResource } from "./types";