fix(http/file_server): Redirect non-canonical URL to canonical URL

[ayame113]

part of #3361

This PR changes the serveDir()'s behavior so that URLs are canonicalized.
The current behavior and the behavior after the change are as follows.

request	before	after
/path/to/directory	🟢200 OK	🟡301 redirect to /path/to/directory/
/path/////to/////directory///////	🟢200 OK	🟡301 redirect to /path/to/directory/
/path/////to/////file.txt	🟢200 OK	🟡301 redirect to /path/to/file.txt
/path/to/file.txt/	🔴500 Internal Server Error	🟡301 redirect to /path/to/file.txt
/path/////to/////file.txt/	🔴500 Internal Server Error	🟡301 redirect to /path/to/file.txt
/path/////to/////file.txt/////	🔴500 Internal Server Error	🟡301 redirect to /path/to/file.txt

Also serveDir() currently has 7 nestings (function-try-if-if-try-if-if). I cut this part out into a separate function.

[kt3k]

New redirect cases make sense to me. Also this looks nice clean up! LGTM