Description of the Change

Adds root mu-plugins to the gitignore so contributors can conveniently map some local mu-plugins.

How to test the Change

1. Create a .wp-env.override.json with

{
	"$schema": "https://schemas.wp.org/trunk/wp-env.json",
	"mappings": {
		"wp-content/mu-plugins": "./mu-plugins"
	}
}

2. Make a mu-plugins directory in the root and throw a drop-in plugin in there.
3. The plugin should load after restarting the WordPress environment, and it won't be tracked in the repo.

Changelog Entry

Changed - Add mu-plugins/ to .gitignore for mappings in potential .wp-env.override.json

Credits

@JordanPak