Minor performance improvements

[tiny-ben-tran]

Summary

• Replace stringMatch with regExpTest since the latter is faster (https://stackoverflow.com/a/10940138/11275515)
• Move commonSvgAndHTMLElements out so _checkValidNamespace() function doesn't re-create it in every single call
• mXSS detection to bypass empty node

Background & Context

The engineers at Tiny made some improvements to the DOMpurify and would like to make a contribution upstream so we don't have to maintain these.

[cure53]

This looks good, thank you! 😄