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

scripts/build-if-not-git-repo.js

@@ -1,4 +1,3 @@
-/* eslint-disable @typescript-eslint/no-var-requires */
 /* eslint-env node */
 
 // Runs a webpack build when not installed in a git repository (i.e. when installed from GitHub in node_modules)
@@ -26,7 +25,7 @@ const git = (args) => cp.spawnSync("git", args, { stdio: "inherit" });
 // Ensure that we're inside a Git repository
 // If git command is not found, status is null and we should return
 // That's why status value needs to be checked explicitly
-if (git(["rev-parse"]).status !== 0) {
+if (git(["rev-parse"]).status !== 0 || process.argv.includes("--force")) {
   l(`git command not found, running build`);
   const compiler = webpack(config.map((c) => c()));