Add a remove button to the link tooltip

Uses `@quill.editor.doc.findLeafAt(range)` to expand the selection to include the entire contents of the anchor (assuming that the leaf found is the anchor). This assumption makes me kind of nervous... but I couldn't come up with a better way to do this.

Clicking the remove button in the tooltip always removes the link from the entire leaf.

Clicking the button in the WYSIWYG bar can remove the link from just the selected text (if the collection is not collapsed), otherwise it acts similarly to the tooltip remove button.