Make ignoring modifiers on IME input optional

By default, ignore modifier keys on IME input, but add
a variable to get back old behavior.
* src/w32fns.c (syms_of_w32fns): New variable
w32-ignore-modifiers-on-IME-input.
(w32_wnd_proc): Use it to ignore modifier keys when IME input is
used.  (Bug#44641)

* etc/NEWS: Announce the change and the new variable.
1 job for emacs-27 in 111 minutes and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #8922
test-all

01:51:16