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