fix: make onDOMBeforeInput optional (#3138)

Fix this typescript error:

```
Property 'onDOMBeforeInput' is missing in type '{}' but required in type ...
```

with this code:
```
<Slate editor={editor} defaultValue={defaultValue}>
  <Editable />
</Slate>
```

packages/slate-react/src/components/editable.tsx

@@ -38,7 +38,7 @@ import {
 
 export const Editable = (props: {
   decorate?: (entry: NodeEntry) => Range[]
-  onDOMBeforeInput: (event: Event) => void
+  onDOMBeforeInput?: (event: Event) => void
   placeholder?: string
   readOnly?: boolean
   role?: string