Add todo.txt mime type
Adds mime type for the todo.txt file spec defined in https://github.com/todotxt/todo.txt
Relevant discussion about this format: https://gitlab.gnome.org/GNOME/gtksourceview/-/merge_requests/315#note_1694100
Adds mime type for the todo.txt file spec defined in https://github.com/todotxt/todo.txt
Relevant discussion about this format: https://gitlab.gnome.org/GNOME/gtksourceview/-/merge_requests/315#note_1694100