[FIX] web_editor: use Node from editable's document

Steps to reproduce:
- In website, paste a URL like "odoo.com" as plain text.

The expected result is a transformed link, just like it happens when
pasting is done in an html field (ex: project.task description), but
a simple text node is inserted instead.

The reason for this is that an element created by the iframe document's
`createElement` method has in its prototype chain the iframe context's
global `Node` constructor, which is a distinct constructor object from the
main window's `Node` constructor.

task-3521152

closes odoo/odoo#136545

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>