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/url-resources.ts

@@ -6,15 +6,8 @@ import { getReplayLink } from "./pageutils";
 import fasSearch from "@fortawesome/fontawesome-free/svgs/solid/search.svg";
 
 import "keyword-mark-element/lib/keyword-mark.js";
-import { type Item as ItemType } from "./types";
-
-type URLResource = {
-  url: string;
-  mime: string;
-  status: string;
-  date: string;
-  ts: string;
-};
+import { type ItemType } from "./types";
+import { type URLResource } from "./types";
 
 // ===========================================================================
 /**