[GR-18163] Use a more portable way to check if code is parseable in the rdoc default gem

PullRequest: truffleruby/3917

Author: andrykonchin

Diff for: lib/mri/rdoc/markup/to_html.rb

@@ -426,7 +426,10 @@ def list_end_for(list_type)
 
   def parseable? text
     verbose, $VERBOSE = $VERBOSE, nil
-    eval("BEGIN {return true}\n#{text}")
+    catch(:valid) do
+      eval("BEGIN { throw :valid, true }\n#{text}")
+      false
+    end
   rescue SyntaxError
     false
   ensure