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/misc.ts

@@ -1,10 +1,9 @@
 "use strict";
 
-import { LitElement, html, css, unsafeCSS } from "lit";
+import { LitElement, html, css, unsafeCSS, type CSSResultGroup } from "lit";
 import { unsafeSVG } from "lit/directives/unsafe-svg.js";
 import { styleMap } from "lit/directives/style-map.js";
 
-// @ts-expect-error - TS2307 - Cannot find module '../assets/main.scss' or its corresponding type declarations.
 import allCssRaw from "../assets/main.scss";
 
 import rwpLogo from "../assets/logo.svg";
@@ -14,7 +13,7 @@ const replayPrefix = "./w";
 
 // ===========================================================================
 const allCss = unsafeCSS(allCssRaw);
-function wrapCss(custom) {
+function wrapCss(custom: CSSResultGroup): CSSResultGroup {
   return [allCss, custom];
 }
 
@@ -22,7 +21,7 @@ const IS_APP =
   // @ts-expect-error - TS2339 - Property 'IS_APP' does not exist on type 'Window & typeof globalThis'.
   window.IS_APP ||
   // @ts-expect-error - TS2551 - Property 'electron' does not exist on type 'Window & typeof globalThis'. Did you mean 'Electron'? | TS2551 - Property 'electron' does not exist on type 'Window & typeof globalThis'. Did you mean 'Electron'?
-  (window.electron && window.electron.IS_APP) ||
+  window.electron?.IS_APP ||
   window.matchMedia("(display-mode: standalone)").matches;
 
 const VERSION = __VERSION__;