Fix several todo-mode.el item editing bugs (bug#63811)

Fix several todo-mode.el item editing bugs (bug#63811)

* lisp/calendar/todo-mode.el (todo-insert-item--basic): With
insertion type 'here', ensure item is inserted on the todo-mode
line where the command was invoked.
(todo-edit-item--cat, todo-edit-item--pos): New variables.
(todo-edit-item--text): Restrict the scope of nil-valued
buffer-read-only to the functions that change buffer text.  If
user moved point while editing a single-line todo item or a done
item comment, or while inserting a done item comment, restore
point, and for comments, make sure the done items section is
displayed.  For multiline items, set the new variables so
todo-edit-quit can use them.
(todo-edit-quit): Use the values of the new variables to restore
point in the todo-mode buffer if it had been moved while editing.
(todo-edit-item--header): Avoid clobbering match data when editing
a todo item header.