[FIX] web_editor: load odooEditor base style

Before this commit
The style for `li.oe-nested` was loaded only when the editor loaded.
The problem is that the style needs to be visible even when the editor
has never been loaded.

After this commit
The style for `li.oe-nested` is always loaded in the backend and in
the frontend.

Task-2793528

closes odoo/odoo#86758

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