Add <slot> (web-platform-dx#622)

Author: foolip

feature-group-definitions/slot.yml

@@ -0,0 +1,18 @@
+name: <slot>
+spec: https://html.spec.whatwg.org/multipage/scripting.html#the-slot-element
+usage_stats: https://chromestatus.com/metrics/feature/timeline/popularity/1898
+compat_features:
+  - api.Element.assignedSlot
+  - api.Element.slot
+  - api.HTMLSlotElement
+  - api.HTMLSlotElement.assign
+  - api.HTMLSlotElement.assignedElements
+  - api.HTMLSlotElement.assignedNodes
+  - api.HTMLSlotElement.name
+  - api.HTMLSlotElement.slotchange_event
+  - api.ShadowRoot.slotAssignment
+  - api.Text.assignedSlot
+  - css.selectors.slotted
+  - html.elements.slot
+  - html.elements.slot.name
+  - html.global_attributes.slot