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.

tsconfig.json

@@ -44,6 +44,7 @@
     "stripInternal": true,
     "verbatimModuleSyntax": true,
     "types": [
+      "webpack/module",
       "vitest/globals",
       "./web_src/js/globals.d.ts",
       "./types.d.ts",