[IMP] portal: add support for .closest modifier

It is sometimes useful in practice to be able to configure the portal so
that it looks for a target as close as possible as the portal location.
For example, in odoo, a portal may be set up in a form view that should
target the current form view. But if the form view itself is in a
dialog, it may fail, because it may find a valid target in the form view that is located
underneath.

With this commit, we add a .closest modifier to the `t-portal`
directive.

doc/reference/portal.md

@@ -15,3 +15,11 @@ class SomeComponent extends Component {
 The `t-portal` directive takes a valid css selector as argument. The content of
 the portalled template will be mounted at the corresponding location. Note that
 Owl need to insert an empty text node at the location of the portalled content.
+
+The `t-portal` directive supports a `.closest` modifier. It is useful to select
+the closest target from the portal location: Owl will look for a target in the
+current parent element, then in its parent, and so on until it finds it.
+
+```xml
+      <div t-portal.closest="'.target'">some content</div>
+```