fix(edit.c): indentkeys double indent after "!" #12894

which is both unexpected and different from the Vim behaviour.

Indent was triggered once by the '!' check in insert_execute(), and
inserting the char was correctly skipped, but then triggered again in
insert_check() (provided that cindent was not being ignored after manual
indentation, i.e. `can_cindent == true`).

While this is the smallest fix, another solution would be to remove
VimState#check and instead move that to *_enter()/-_execute(), since the
control flow is pretty unnecessarily convoluted as is. That would also
have the benefit of differing less from the Vim source code.

test/functional/editor/mode_insert_spec.lua

@@ -6,12 +6,20 @@ local expect = helpers.expect
 local command = helpers.command
 local eq = helpers.eq
 local eval = helpers.eval
+local curbuf_contents = helpers.curbuf_contents
 
 describe('insert-mode', function()
   before_each(function()
     clear()
   end)
 
+  it('indents only once after "!" keys #12894', function()
+    command('let counter = []')
+    command('set indentexpr=len(add(counter,0))')
+    feed('i<C-F>x')
+    eq(' x', curbuf_contents())
+  end)
+
   it('CTRL-@', function()
     -- Inserts last-inserted text, leaves insert-mode.
     insert('hello')