Dead code plugin for `sorbet` should consider RBS comments for `override`

[Morriar]

During dead code detection we currently ignore methods marked as override in their sig block so this method won't be marked as dead:

sig { override.void }
def foo; end

Now that we migrated to RBS signature comments, the equivalent signature is considered dead:

# @overrride
#: -> void
def foo; end

We'll need to consider RBS signatures comments in this plugin.