[FIX] web_editor: handle `default_focus` form attribute for html_field

When rendering a form view that has a truthy `default_focus` attribute,
the renderer looks for the element in its tree that has its `name` as
`id`. However in the case of `html_field` it couldn't find it since
`html_field` failed to apply that `id` to its focusable element (the
editable root). This adds the `id` to the editable root when rendering
the element in `wysiwyg`, and also directly applies `contenteditable` to
it so the `focus` method actually does what we expect it to, even though
we are waiting for `HtmlFieldWysiwygAdapterComponent` to be mounted
before actually instantiating the editor itself.

task-3073096

Part-of: odoo/odoo#106537