Improve errors & warnings due to fancy quoted vars (Bug#32939)

Add some hints to the message for byte compiler free & unused variable
warnings, and 'void-variable' errors where the variable has confusable
quote characters in it.
* lisp/help.el (uni-confusables), uni-confusables-regexp): New
constants.
(help-command-error-confusable-suggestions): New function, added to
`command-error-function'.
(help-uni-confusable-suggestions): New function.
* lisp/emacs-lisp/bytecomp.el (byte-compile-variable-ref):
* lisp/emacs-lisp/cconv.el (cconv--analyze-use): Use it.

* lisp/emacs-lisp/lisp-mode.el
(lisp--match-confusable-symbol-character): New function.
(lisp-fdefs): Use it to fontify confusable characters with
font-lock-warning-face when they occur in symbol names.
* doc/lispref/modes.texi (Faces for Font Lock):
* doc/lispref/objects.texi (Basic Char Syntax): Recommend backslash
escaping of confusable characters, and mention new fontification.
* etc/NEWS: Announce the new fontification behavior.
* test/lisp/emacs-lisp/lisp-mode-tests.el (lisp-fontify-confusables):
New test.
1 job for master in 56 minutes and 28 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #4378
test-all

00:56:28

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
Makefile:294: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1