Keep cursor unlinked to avoid elements being pushed down inside

[luin]

When .ql-cursor is inside a link, and the user types a character, Safari will push down the link inside .ql-cursor and move the cursor's textNode inside the link. This breaks our position calculation. So this PR makes sure the cursor is always unlinked to avoid this.

To test:

1. Type "abc", bold them and linkify them.
2. Move the caret after "c", press cmd+b to cancel the bold format.
3. Type any characters, and delete some characters, make sure everything works as expected.