Preserve the selection visually when changing the link URL

[ngryman]

When the user focuses the URL field to update the link URL, the editor loses focus visually. This could be handled by subscribing to the editor blur event and highlighting the current selection with a new mark. When the editor has focused again, the mark is removed. A generic style extension could be created for that purpose (cf. https://github.com/ueberdosis/tiptap/blob/develop/packages/extension-text-style/src/text-style.ts).