Support querying modification timestamps (`mtime`) for the Deploy filesystem

I recently noticed that `std/http/file_server.ts` in Deploy doesn't add `Last-Modified` or `ETag` headers, which results in clients having to redownload the files when they might already have them cached. This seems to be because the Deploy filesystem doesn't support querying the modification timestamp ([`mtime`](https://deno.land/api@v1.30.3?s=Deno.FileInfo#prop_mtime)) of files.

Even returning the timestamp of the latest deployment would be a useful improvement in this regard, although of course it would be much better to return the timestamp of the first deployment that includes the commit that last modified the given file.