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-receipt.ts

@@ -10,7 +10,7 @@ import { LitElement, html, css, nothing } from "lit";
 import { tsToDate } from "./pageutils";
 import prettyBytes from "pretty-bytes";
 import { property } from "lit/decorators.js";
-import type { Item as ItemType } from "./types";
+import type { ItemType } from "./types";
 
 // ===========================================================================
 export class RWPEmbedReceipt extends LitElement {
@@ -183,7 +183,7 @@ export class RWPEmbedReceipt extends LitElement {
       datapackageHash,
       publicKey,
       software,
-    } = this.collInfo?.verify || {};
+    } = this.collInfo?.verify ?? {};
 
     const sourceUrl = this.collInfo?.sourceUrl;