Provide some way to disable rules provided

[mglaman]

Currently, to use this project and only do deprecation testing one would need to add the following to their phpstan.neon:

	# Ignore phpstan-drupal extension's rules.
	ignoreErrors:
		- '#\Drupal calls should be avoided in classes, use dependency injection instead#'
		- '#Plugin definitions cannot be altered.#'
		- '#Missing cache backend declaration for performance.#'
		- '#Plugin manager has cache backend specified but does not declare cache tags.#'

It would be nice if there was an easier way to kill this as the rules grow.

[mglaman]

Can copy what strict-rules is providing.

[mglaman]

Dupe of #385