Enable yanking the last sexp to a REPL prompt

* sly-mrepl.el (sly-mrepl--with-repl-for): Add edebug spec.
(sly-mrepl--eval-for-repl): Make into cl-defun with keyword args.
(sly-mrepl--copy-objects-to-repl): Use it.
(sly-mrepl-sync): Renamed
from `sly-mrepl-sync-package-and-default-directory'. Add new
interactive argument. Yank last expression into the REPL.

* NEWS.md (mREPL enhancements): Mention `sly-mrepl-sync' changes.