[FIX] web_editor: properly clean zero-width spaces inserted for formats

When inserting a format with a collapsed selection, we need to insert a
zero-width space. It should not be saved so we need to isolate it and
put the "oe-zws-empty-inline" attribute on its parent so it gets cleaned
up by the sanitizer.

X-original-commit: 4ce274ba39cd8046b9ef4e0b99df66913375f993
Part-of: odoo/odoo#86930