[ruby/rdoc] Use a more portable way to check if code is parseable

* The same as used in irb: https://github.com/ruby/irb/pull/134/files
* This works on all Ruby implementations, unlike `return` in BEGIN which
  can be quite difficult to support.

https://github.com/ruby/rdoc/commit/d19f7c66fe

lib/rdoc/markup/to_html.rb

@@ -430,7 +430,9 @@ class RDoc::Markup::ToHtml < RDoc::Markup::Formatter
 
   def parseable? text
     verbose, $VERBOSE = $VERBOSE, nil
-    eval("BEGIN {return true}\n#{text}")
+    catch(:valid) do
+      eval("BEGIN { throw :valid, true }\n#{text}")
+    end
   rescue SyntaxError
     false
   ensure