Make edebug see unused variables when lexical-binding is non-nil

Make edebug see unused variables when lexical-binding is non-nil

This fixes bug #59213.

* lisp/emacs-lisp/cconv.el (cconv-dont-trim-unused-variables): New variable.
(cconv-fv, cconv-make-interpreted-closure): Add/amend doc strings.
(cconv-make-interpreted-closure): Test cconv-dont-trim-unused-variables, and
if non-nil, don't "optimize" the lexical environment.

* lisp/emacs-lisp/edebug.el (edebug-make-enter-wrapper): Compile a binding of
cconv-dont-trim-unused-variables to t around the call of edebug-enter.

* lisp/emacs-lisp/testconver.el (testcover-analyze-coverage): Add a new arm to
the pcase form to handle the new form of edebug-enter.