[3.14] gh-137836: Support more RAWTEXT and PLAINTEXT elements in HTMLParser (GH-137837) (GH-140841)

* the "plaintext" element
* the RAWTEXT elements "xmp", "iframe", "noembed" and "noframes"
* optionally RAWTEXT (if scripting=True) element "noscript"
(cherry picked from commit a17c57eee5b5cc81390750d07e4800b19c0c3084)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>