[IMP] slots: add new t-set-slot directive

This new t-set-slot directive is meant to replace t-set when we need to
define the content of a sub slot. All new code should use that
directive.

The old t-set directive is still supported for now, but this should be
removed when we publish Owl 2.0.

roadmap.md

@@ -16,6 +16,8 @@ change!
 
 ### 2.x (2020? 2021? 2022?)
 
+- stop support for `t-set` directive to define the content of a slot
+
 Maybe:
 
 - reimplement vdom to use *block* system, like Vue 3, which should make Owl