patch 9.0.2015: Vim9: does not handle islocked() from a method correctly

patch 9.0.2015: Vim9: does not handle islocked() from a method correctly

Problem:  Vim9: does not handle islocked() from a method correctly
Solution: Handle islocked() builtin from a method.

- Setup `lval_root` from `f_islocked()`.
- Add function `fill_exec_lval_root()` to get info about executing method.
- `sync_root` added in get_lval to handle method member access.
- Conservative approach to reference counting.

closes: #13309

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>