[IMP] website: auto-discover all available fonts in the theme modal

Before this commit, if a theme had more fonts that the default number
(6), it had to override the customize dialog. Now, the number of fonts
to add in the list is found automatically.

To do so, instead of defining a <selection/> tag with 6+ <opt/> tag, we
are now able to define an unique new tag: <fontselection/> which will do
automatically the job.

Part of https://github.com/odoo/odoo/pull/33442
task-1974659