add .mjs to supported extensions

[csorrentino]

This PR adds .mjs to the default SUPPORTED_EXTENSIONS. Wordpress packages like @wordpress/icons switched to .mjs for their builds in a recent version.

The plugin's transform hook skips .mjs files, so the @wordpress/primitives imports inside @wordpress/icons are never rewritten to use wp.primitives globals.

The problem can easily be replicated using a fresh Radicle project because @wordpress/icons is used in a few of the included blocks. Without adding .mjs as a supported extension the browser fails with:

Uncaught TypeError: Failed to resolve module specifier "@wordpress/primitives". 
Relative references must start with either "/", "./", or "../".

Because wordpress packages seem to be moving to using .mjs extensions, I thought this would be a helpful default, but this can also be easily solved at the project level by adding the extension to the vite.config.js

[retlehs]

Thanks for finding this and for the PR!