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

docs/concepts/07-editor.md

@@ -12,6 +12,7 @@ interface Editor {
   // Schema-specific node behaviors.
   isInline: (element: Element) => boolean
   isVoid: (element: Element) => boolean
+  markableVoid: (element: Element) => boolean
   normalizeNode: (entry: NodeEntry) => void
   onChange: () => void
   // Overrideable core actions.
@@ -70,6 +71,20 @@ editor.insertText = text => {
 }
 ```
 
+If you have void "mention" elements that can accept marks like bold or italic:
+
+```javascript
+const { isVoid, markableVoid } = editor
+
+editor.isVoid = element => {
+  return element.type === 'mention' ? true : isInline(element)
+}
+
+editor.markableVoid = element => {
+  return element.type === 'mention' || markableVoid(element)
+}
+```
+
 Or you can even define custom "normalizations" that take place to ensure that links obey certain constraints:
 
 ```javascript