Use Rust-based parser for tasklist parsing

Fixes Use native task list parsing code (comrak), and... (#357684 - closed). See also previous draft at Draft: Use native task list parsing in `comrak`... (!198758 - closed); we have since updated the Rust-based parser to correctly deal with Unicode whitespace.

Updates our gitlab-glfm-markdown dependency to 0.0.39, which includes the Unicode whitespace fix. — this update has since been made in master already.