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/+/169502Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/mime/type.go

@@ -68,6 +68,7 @@ var builtinTypesLower = map[string]string{
 	".png":  "image/png",
 	".svg":  "image/svg+xml",
 	".xml":  "text/xml; charset=utf-8",
+	".mjs":  "text/javascript",
 }
 
 var once sync.Once // guards initMime