Bump master Emacs version to 30.0.50

Bump master Emacs version to 30.0.50

* README:
* configure.ac:
* nt/README.W32:
* msdos/sed2v2.inp:
* etc/refcards/ru-refcard.tex: Bump Emacs version to 30.0.50.

* etc/NEWS.29: New file.
* etc/NEWS: Empty for Emacs 30.

* configure.ac: Add boilerplate for module-env-XX.h.

* src/emacs-module.h.in (struct emacs_env_30): Add.
* src/module-env-29.h: Remove header comment.
* src/module-env-30.h: New file.

* lisp/cus-edit.el (customize-changed-options-previous-release):
Set last version to 29.1.