Introduce and use minibuffer-mode. This fixes bug #47150

Introduce and use minibuffer-mode.  This fixes bug #47150

* lisp/minibuffer.el (minibuffer-mode): New derived mode.

* src/minibuf.c (syms_of_minibuf): New DEFSYMs Qminibuffer_mode,
Qminibuffer_inactive_mode, Qminibuffer_completing_file_name,
Qselect_frame_set_input_focus, Qadd_to_history.
(read_minibuf, set_minibuffer_mode, read_minibuf_unwind): Use the new DEFSYMs
in place of continual interning.
(set_minibuffer_mode): Put an active minibuffer into minibuffer-mode rather
than fundamental-mode.

* doc/emacs/mini.texi (Minibuffer Edit): Mention minibuffer-mode.

* doc/lispref/minibuf.texi (Intro to Minibuffers): Add a paragraph about
minibuffer-mode.

* etc/NEWS (Incompatible Lisp Changes in Emacs 28.1): Add an entry.