Add initial RBS support with Steep and TypeProf

sig/vendor/event_emitter.rbs

@@ -0,0 +1,9 @@
+# Added only to satisfy the Steep requirements. Not 100% reliable.
+module EventEmitter
+  def add_listener: (untyped `type`, ?{once: true} params) -> Integer
+  alias on add_listener
+
+  def remove_listener: (untyped id_or_type) -> Array[untyped]?
+  def emit: (untyped `type`, *untyped data) -> Array[untyped]
+  def once: (untyped `type`) -> Integer
+end