Make warning about unescaped character literals more helpful.

Make warning about unescaped character literals more helpful.

See Bug#31676.

* lisp/emacs-lisp/byte-run.el
(byte-run--unescaped-character-literals-warning): New defun.

* src/lread.c (load_warn_unescaped_character_literals): Use new defun.
(syms_of_lread): Define symbol for new defun.

* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Use new
defun.

* test/src/lread-tests.el (lread-tests--unescaped-char-literals):
test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--unescaped-char-literals): Adapt unit tests.