Add mjs file extension for git style subcommands

[eduardbadillo]

Hi,

I'm sending this PR to add support for .mjs files when executing git style subcommands. The explanation for this type of files can be found here: https://nodejs.org/api/esm.html

Following the Readme file, I've setup some code to run a basic cli program. I have changed the shebang in my files for the following, so I'm able to use JS modules in node:
#!/usr/bin/env node --experimental-modules --no-warnings .

This adds the ability to find .mjs as executable files, as well as the previous .ts and .js.

Please, let me know if something else is required.

Thank you

[shadowspawn]

Possible release note:

Add ECMAScript Module suffix .mjs for git-style sub-commands

[shadowspawn]

Looks good to me.

(Simple and same pattern as existing code.)