fix: add condition for using steep and typeprof to pass CI with all Ruby versions

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.

Gemfile

@@ -5,6 +5,13 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in youplot.gemspec
 gemspec
 
+group :development do
+  if RUBY_VERSION >= '3.0'
+    gem 'steep', require: false
+    gem 'typeprof'
+  end
+end
+
 group :test do
   gem 'rake'
   gem 'simplecov'

Steepfile

@@ -0,0 +1,8 @@
+D = Steep::Diagnostic
+
+target :lib do
+  signature "sig"
+  check "lib" # Directory name to check
+
+  configure_code_diagnostics(D::Ruby.silent) # `silent` diagnostics setting
+end

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