fix(aio): do not fallback to `index.html` for file requests (#15401)

Previously, all URLs were rewritten to `index.html` in order to support
deep-linking. This works when navigating to URLs that correspond to existing
resources. E.g. navigating to `/tutorial` returns `index.html` and then the
`DocViewer` takes over and requests `tutorial.json`.
Navigating to a non-existent URL (e.g. `/foo`), will return `index.html`, which
in turn requests (the non-existent) `foo.json` and throws an error when trying
to parse the returned `index.html` as JSON.

This commit fixes it by only rewriting URLs that do not request a file (i.e. do
not include a `.` in the last path segment).

Fixes #15398

aio/firebase.json

@@ -7,7 +7,7 @@
     "cleanUrls": true,
     "rewrites": [
       {
-        "source": "**",
+        "source": "**/!(*.*)",
         "destination": "/index.html"
       }
     ]