Always heed the `lexical-binding' local variable

Always heed the `lexical-binding' local variable

* doc/lispref/variables.texi (File Local Variables): Document
`permanently-enabled-local-variables'.

* lisp/files.el (enable-local-variables): Mention the new variable.
(set-auto-mode): Always call `hack-local-variables'.
(hack-local-variables): Factor out the variable gathering into its
own function, and respect the new variable (bug#47843).
(hack-local-variables--find-variables): Factored out from
`hack-local-variables'.
(permanently-enabled-local-variables): New variable.