mime: update .mjs MIME type from text/ to application/javascript
.mjs should be the same MIME type as .js, and RFC 4329 says that
text/javascript is obsolete, even back in 2006:
https://tools.ietf.org/html/rfc4329#section-7.1
I didn't notice this when I recently reviewed CL 169502.
Also, re-sort it.
Updates #30547
Change-Id: I8ed8ddaf06c8a08b010423ebd071f39ef3a325e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/175459
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>