fix(api): disallow some more functions during textlock

Problem: nvim_buf_set_text(), nvim_open_term() and termopen() all change buffer
text, which is forbidden during textlock. Additionally, nvim_open_term() and
termopen() may be used to convert the cmdwin buffer into a terminal buffer,
which is weird.

Solution: Allow nvim_buf_set_text() and nvim_open_term() in the cmdwin, but
disallow nvim_open_term() from converting the cmdwin buffer into a terminal
buffer. termopen() is not allowed in the cmdwin (as it always operates on
curbuf), so just check text_locked().

Also happens to improve the error in #21055: nvim_buf_set_text() was callable
during textlock, but happened to check textlock indirectly via u_save();
however, this caused the error to be overwritten by an unhelpful "Failed to
save undo information" message when msg_list == NULL (e.g: an `<expr>` mapping
invoked outside of do_cmdline()).

test/functional/terminal/buffer_spec.lua

@@ -498,3 +498,17 @@ if is_os('win') then
     end)
   end)
 end
+
+describe('termopen()', function()
+  before_each(clear)
+
+  it('disallowed when textlocked and in cmdwin buffer', function()
+    command("autocmd TextYankPost <buffer> ++once call termopen('foo')")
+    matches("Vim%(call%):E565: Not allowed to change text or change window$",
+      pcall_err(command, "normal! yy"))
+
+    feed("q:")
+    eq("Vim:E11: Invalid in command-line window; <CR> executes, CTRL-C quits",
+      pcall_err(funcs.termopen, "bar"))
+  end)
+end)