Handle thread-signal towards the main thread (Bug#32502)

Handle thread-signal towards the main thread  (Bug#32502)

* doc/lispref/threads.texi (Basic Thread Functions):
* etc/NEWS: Document thread-signal towards the main thread.

* lisp/emacs-lisp/thread.el: New package.

* src/keyboard.c (read_char): Check for Qthread_event.
(kbd_buffer_get_event, make_lispy_event): Handle THREAD_EVENT.
(syms_of_keyboard): Declare Qthread_event.
(keys_of_keyboard): Add thread-handle-event to special-event-map.

* src/termhooks.h (enum event_kind): Add THREAD_EVENT.

* src/thread.c: Include "keyboard.h".
(poll_suppress_count) Don't declare extern.
(Fthread_signal): Raise event if THREAD is the main thread.  (Bug#32502)

* test/src/thread-tests.el (thread): Require it.
(threads-signal-main-thread): New test.