Restrict symbol prettification to displayable glyphs

Restrict symbol prettification to displayable glyphs

* lisp/international/mule.el (char-displayable-on-frame-p): New
function used to determine whether a character can be
meaningfully displayed on a given frame.
* doc/lispref/display.texi (Fontsets): Document it.
* lisp/progmodes/prog-mode.el
(prettify-symbols--composition-displayable-p): New function used
to restrict to displayable prettification symbols.  This
prevents issues with missing characters appearing as boxes.
(prettify-symbols--make-keywords): Use it.  (Bug#77381)

(cherry picked from commit 2d0b5f34a008979d34f337c872bcf93a296c6ec2)