(lexical-binding): Allow changing the default value (bug#74145)

(lexical-binding): Allow changing the default value (bug#74145)

* lisp/loadup.el: Preserve (default-toplevel-value 'lexical-binding).

* src/lread.c (Fload, Feval_buffer): Obey (default-toplevel-value
'lexical-binding).

* doc/lispintro/emacs-lisp-intro.texi (Lexical vs Dynamic Binding
Example): Use the lexical dialect also for the dynamic
scoping example.
* doc/lispref/edebug.texi (Edebug Eval): Remove long-obsolete mention
of Edebug support for `lexical-let`.

(cherry picked from commit d685d21e8ae0cee566420d4dd68586f018b2ce0b)