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/electron-replay-app.ts

@@ -250,7 +250,14 @@ class ElectronReplayApp {
     return rv;
   }
 
-  async doIntercept(request, callback) {
+  async doIntercept(
+    request,
+    callback: (response: {
+      statusCode: number;
+      headers: Record<string, string>;
+      data: fs.ReadStream;
+    }) => void,
+  ) {
     console.log(`${request.method} ${request.url} from ${request.referrer}`);
 
     // if local server
@@ -303,7 +310,7 @@ class ElectronReplayApp {
     }
 
     // possible 'live leak' attempt, return archived version, if any
-    if (request.referrer && request.referrer.startsWith(REPLAY_PREFIX)) {
+    if (request.referrer?.startsWith(REPLAY_PREFIX)) {
       return await this.resolveArchiveResponse(request, callback);
     }
 
@@ -483,7 +490,6 @@ class ElectronReplayApp {
   }
 
   getOpenUrl(argv) {
-    // eslint-disable-next-line @typescript-eslint/no-var-requires
     argv = require("minimist")(argv.slice(process.defaultApp ? 2 : 1));
 
     const filename =