Use JavaScript syntax highlighting on .mjs files

Created by: emchap

Feature request description

Apply the JS syntax highlighting to files with the .mjs extension. It's the same syntax and today the files have no syntax highlighting.

Is your feature request related to a problem? If so, please describe.

Customer requested this after noticing it when looking for OSS code on Dotcom, but this is true on all instances. (Context.)

Describe alternatives you've considered.

Suggested that customer set up extension mapping locally, but they ran into this issue on Dotcom.

Additional context

See thread. @tjdevries said he'd take a look when he had some cycles.