Support for `.mjs` files

[scur-iolus]

I've been using this action in my project and noticed that it currently doesn't minify .mjs files. With the increasing use of ES modules, .mjs is becoming more common, and it would be great if the action could support minifying them as well.

Please add support for .mjs files in the minification process, similar to how .js files are currently handled.

Thanks for creating and maintaining this useful action!