Add default support for mjs files

[melink14]

mjs is the accepted file extension for JS with modules so it makes sense to include it in the default set of extensions checked with gts check.

I tried to support it via commandline flags but it looks like there is no way to extend the set of supported filetypes via gts flags since adding any commandline arguments overwrites the default ones.

[RyanMartin-Carewell]

Should likely support .cjs as well, then. And .mts and .cts for the TypeScript equivalents