feat: add processing.rbs as a type definition file for processing.rb

As a first step toward adopting rbs for this project, added type a definition for a simple method.
More type definitions will be added incrementally.

Author: ericgpks

sig/youplot/backends/processing.rbs

@@ -0,0 +1,7 @@
+module YouPlot
+  module Backends
+    module Processing
+      def count_values: ([], ?tally: true, ?reverse: false) -> Array[Array[untyped]]
+    end
+  end
+end