Fix several todo-mode.el editing bugs (bug#42976)

Fix several todo-mode.el editing bugs (bug#42976)

* lisp/calendar/todo-mode.el (todo-insert-item--basic): Ensure the
target todo file is in todo-mode.
(todo-edit-item--text): When editing a done item comment, prevent
clobbering match data on finishing the edit.
(todo-edit-item--header): Ensure that decrementing the month of
the date header works for intervals greater than a year, and when
incrementing or decrementing the month crosses one or more years,
adjust the year as needed.
(todo-read-category): If we're outside of todo-mode and there is a
current todo file, use it; otherwise, use the default todo file.

* test/lisp/calendar/todo-mode-tests.el
(todo-test-edit-item-date-month): New test.

* test/lisp/calendar/todo-mode-resources/todo-test-1.todo: Modify
to accommodate new test.