Add pattern matching pin support for instance/class/global variables

Pin matching for local variables and constants is already supported,
and it is fairly simple to add support for these variable types.

Note that pin matching for method calls is still not supported
without wrapping in parentheses (pin expressions).  I think that's
for the best as method calls are far more complex (arguments/blocks).

Implements [Feature #17724]

test/ruby/test_pattern_matching.rb

@@ -400,6 +400,30 @@ END
         a == 0
       end
     end
+
+    assert_block do
+      @a = /a/
+      case 'abc'
+      in ^@a
+        true
+      end
+    end
+
+    assert_block do
+      @@TestPatternMatching = /a/
+      case 'abc'
+      in ^@@TestPatternMatching
+        true
+      end
+    end
+
+    assert_block do
+      $TestPatternMatching = /a/
+      case 'abc'
+      in ^$TestPatternMatching
+        true
+      end
+    end
   end
 
   def test_pin_operator_expr_pattern