Add support for mjs files

[ramykl]

The version of ESLint you are using.
ESLint: 7.31.0

The problem you want to solve.
.mjs files are becoming commonplace and no longer an experimental feature, so I would expect that they would be linted by ESLint (or at least easy to tell ESLint to look at them), this could either be done by setting ESLint to look at .mjs files by default or by adding support to the configuration files to add extensions.

Your take on the correct solution to problem.
I think there are two potential solutions to this problem:

1. include .mjs files in the glob pattern for default linting
2. add extensions support to the top level of the config file to allow adding the custom extensions easily to ESLint

Currently, the only way to get ESLint to run on .mjs files is to pass --ext js,mjs in the command line. Adding this option to the configuration file would help with consistency between the command line API and the package runner.

Are you willing to submit a pull request to implement this change?
Yes - I just couldn't find the correct files (so if someone could point me in the right direction I can attempt to make a PR)

[nzakas]

Already covered by #14588