bug#74415: 29.4; mouse-start-end does not respect syntax-table text properties

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> Thanks.  Guillaume, does the patch below give good results?

the latch LGTM, but it bought up the following side note: maybe we
should add a `syntax-class-of-char` so we can replace

    (= (syntax-class-to-char FOO)
       BAR))

with

    (= FOO
       (syntax-class-of-char BAR))

where `syntax-class-of-char` can be pre-computed during compilation
(when BAR is a constant, which would be always the case in your patch).

Tho, maybe even better would be a `(syntax-class-p BAR FOO)`.

Anyway, nothing urgent.


        Stefan