Add .mjs to default file extentions

[jaydenseric]

This will make using documentation much easier if you have .mjs files in your project:

• The documentation lint command does not find .mjs files, and can not yet be taught to (Unable to specify file extension for linting #1021).
• Running commands like documentation readme requires the use of unintuitive --re mjs --pe mjs arguments (Can't get --extension to work. #891 (comment)).

The .mjs file extension is the official way to run ESM natively in Node.js. All of my packages these days have .mjs files. It is becoming increasingly common in the wild and will eventually be the go-to extension for ES. .mjs is already supported by default by tools such as Webpack.