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/+/175459Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>

src/mime/type.go

@@ -61,15 +61,15 @@ var builtinTypesLower = map[string]string{
 	".gif":  "image/gif",
 	".htm":  "text/html; charset=utf-8",
 	".html": "text/html; charset=utf-8",
-	".jpg":  "image/jpeg",
 	".jpeg": "image/jpeg",
+	".jpg":  "image/jpeg",
 	".js":   "application/javascript",
-	".wasm": "application/wasm",
+	".mjs":  "application/javascript",
 	".pdf":  "application/pdf",
 	".png":  "image/png",
 	".svg":  "image/svg+xml",
+	".wasm": "application/wasm",
 	".xml":  "text/xml; charset=utf-8",
-	".mjs":  "text/javascript",
 }
 
 var once sync.Once // guards initMime