Warn when loading .el files without lexical-binding declaration

Warn when loading .el files without lexical-binding declaration

This corresponds to the byte-compiler warning for the same issue,
here emitted for files that aren't compiled but loaded from source.
It should make the planned change to enable lexical binding by default
in Emacs 31 go smoother.

* src/lread.c (lexical_cookie_t): New type.
(lisp_file_lexically_bound_p): Renamed to...
(lisp_file_lexical_cookie): ...this, with the return value retyped.
* src/lread.c (warn_missing_cookie): New.
(Fload): Warn when loading source file and cookie missing.
(Feval_buffer): Add LOADING arg; warn when set and cookie missing.
* lisp/international/mule.el (load-with-code-conversion):
* lisp/startup.el (command-line--load-script):
Call eval-buffer with LOADING arg set.
* etc/NEWS: Announce.