bug#41031: Add facemenu-set-strong and facemenu-set-emphasis

[積丹尼 Dan Jacobson]

Just the other day I was editing an .html file, and noticed

M-o b           facemenu-set-bold
M-o d           facemenu-set-default
M-o i           facemenu-set-italic
M-o l           facemenu-set-bold-italic
M-o o           facemenu-set-face
M-o u           facemenu-set-underline

hadn't been updated to also include "strong" and "em",
which we are now supposed to use in preference to
"<b>" (bold) and "<i>" (italic), that it currently only inserts.

(Sure, facemenu may have nothing to do with html(-mode). All I know is
that it does insert <b>, so should also insert <strong>.)

emacs-version "26.3"