files.el (major-mode-remap-alist): New custom var (bug#58075)

files.el (major-mode-remap-alist): New custom var (bug#58075)

* lisp/files.el (major-mode-remap-alist): New custom var.
(set-auto-mode--last): New var.
(set-auto-mode-0): Obey `major-mode-remap-alist`.

* doc/emacs/modes.texi (Choosing Modes): Document `major-mode-remap-alist`.

* lisp/progmodes/cperl-mode.el: Recommend the use of
`major-mode-remap-alist` over the crude `defalias` solution.

* lisp/textmodes/tex-mode.el (tex--guess-mode): Simplify.
(tex--redirect-to-submode): Obey `major-mode-remap-alist`.