mime: add javascript module mime type (.mjs)

There are default mime types in this package for handling static content, but there's
a new one missing ".mjs"  that is "Content-Type: text/javascript".

https://developers.google.com/web/fundamentals/primers/modules#mjs

Change-Id: Ie842ece0cb55770fb6c9eb65f8bfee2ecf7bc624
GitHub-Last-Rev: e26d9f76171c987112d5d6db292446819a8393e2
GitHub-Pull-Request: golang/go#31071
Reviewed-on: https://go-review.googlesource.com/c/go/+/169502


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>