Migrate to import.meta.env and clean up types and eslint (#36362)

`import.meta.env` is supported in both vitest and webpack [as of
recent](https://github.com/webpack/webpack/pull/19996), so replace all
previous use of `process.env` with it. Current usage is limited to test
files, I've also verified it works in actual frontend code.

`webpack/module` is added to typescript types which includes the
definition for `import.meta.env`. I've also made the eslint globals more
precise. Finally, `__webpack_public_path__` is removed from our type
definitions because `webpack/module` also provides it.

web_src/js/bootstrap.ts

@@ -6,7 +6,7 @@ import {html} from './utils/html.ts';
 
 // This sets up the URL prefix used in webpack's chunk loading.
 // This file must be imported before any lazy-loading is being attempted.
-__webpack_public_path__ = `${window.config?.assetUrlPrefix ?? '/assets'}/`;
+window.__webpack_public_path__ = `${window.config?.assetUrlPrefix ?? '/assets'}/`;
 
 function shouldIgnoreError(err: Error) {
   const ignorePatterns = [
@@ -41,7 +41,7 @@ export function showGlobalErrorMessage(msg: string, msgType: Intent = 'error') {
 
 function processWindowErrorEvent({error, reason, message, type, filename, lineno, colno}: ErrorEvent & PromiseRejectionEvent) {
   const err = error ?? reason;
-  const assetBaseUrl = String(new URL(__webpack_public_path__, window.location.origin));
+  const assetBaseUrl = String(new URL(window.__webpack_public_path__, window.location.origin));
   const {runModeIsProd} = window.config ?? {};
 
   // `error` and `reason` are not guaranteed to be errors. If the value is falsy, it is likely a