Allow void elements to receive marks (#5135)

Some void elements are effectively stand-ins for text, such as with the mentions example,
where the mention element renders the character's name. Users might want to format Void
elements like this with bold, or set their font and size, so `editor.markableVoid` tells
Slate whether or not to apply Marks to the text children of void elements.

- Adds `markableVoid()` as a schema-specific overrideable test.
- Changes `addMark` and `removeMark` so marks can apply to voids. Also changes behavior
of collapsed selection so that if a markable Void is selected, the mark will be applied /
removed.
- Shows how `markableVoid()` can work in the mentions example

.changeset/modern-cooks-grow.md

@@ -0,0 +1,5 @@
+---
+'slate': patch
+---
+
+Allow void elements to receive marks via markableVoid()