bug#61666: HTML tag selection/matching doesn't work with default Emacs mhtml-mode

[John Cummings]

I was able to recreate this problem on
Emacs 28.2 (debian bookworm)
Hyperbole version either 8.0.0 or 8.0.1pre0.20230217.74322
Emacs major mode: mhtml-mode

Expected behaviors:
Using the Hyperbole action key (M-RET), hui-select-goto-matching-delimiter (C-c 
.), or hui-select-thing (C-c RET), with an HTML tag pair, will perform the 
corresponding actions on the entire tag pair.

Actual behaviors:
The tag pair is not recognized; instead each tag in the pair is just treated 
like a plain sexp delimited by its angle brackets.

Cause:
It looks like a simple cause - mhtml-mode is not in hui-select-markup-modes by 
default. But mhtml-mode is the default major mode for .html files, going at 
least as far back as some Emacs 26 versions, at least in Debian/Ubuntu builds. 
Adding it to this list made things work as expected.