typing: add stub functions for `cext/charencoding`
I'm not sure if it's better to have a separate file, and currently pytype
doesn't really know how to handle these, so it's no help in figuring that out.
Technically, these methods are part of the `mercurial.cext.parsers` module, so
put them into the existing stub until there's a reason to split it out.
# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable