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/electron-preload.ts

@@ -1,8 +1,8 @@
 /*eslint-env node */
 
 import { CollectionLoader } from "@webrecorder/wabac/src/loaders";
+import { type IpcRendererEvent } from "electron";
 
-// eslint-disable-next-line @typescript-eslint/no-var-requires
 const { ipcRenderer, contextBridge } = require("electron");
 
 contextBridge.exposeInMainWorld("electron", { IS_APP: true });
@@ -11,12 +11,7 @@ const dbs = {};
 
 const loader = new CollectionLoader();
 
-/**
- *
- * @param {string} name
- * @returns
- */
-async function getColl(name) {
+async function getColl(name: string) {
   if (!dbs[name]) {
     dbs[name] = await loader.loadColl(name);
     await dbs[name].initing;
@@ -25,25 +20,18 @@ async function getColl(name) {
   return dbs[name];
 }
 
-/**
- * @param {string} name
- * @returns
- */
-async function getDB(name) {
+async function getDB(name: string) {
   const coll = await getColl(name);
   return coll.store;
 }
 
-/**
- *
- * @param {import("electron").IpcRendererEvent} event
- * @param {*} request
- * @param {*} coll
- * @param {*} ts
- * @param {*} channel
- * @returns
- */
-async function getResponse(event, request, coll, ts, channel) {
+async function getResponse(
+  event: IpcRendererEvent,
+  request: Request,
+  coll: string,
+  ts: string,
+  channel: string,
+): Promise<void> {
   const db = await getDB(coll);
 
   const req = { request, url: request.url, timestamp: ts };