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/embed.ts

@@ -2,7 +2,7 @@ import {
   LitElement,
   html,
   css,
-  TemplateResult,
+  type TemplateResult,
   type PropertyValues,
 } from "lit";
 import { ifDefined } from "lit/directives/if-defined.js";
@@ -53,7 +53,6 @@ class Embed extends LitElement {
   @property({ type: String }) newWindowBase = "";
 
   @property({ type: String }) errorMessage:
-    | ""
     | TemplateResult<1>
     | string
     | undefined;
@@ -163,7 +162,7 @@ class Embed extends LitElement {
     }
 
     // if no storage manager or estimate, don't cache
-    if (!navigator.storage || !navigator.storage.estimate) {
+    if (!navigator.storage?.estimate) {
       this.noCache = true;
     }
   }