Fix crash when a void node deletes itself on click

[jameshfisher]

Description

Here's a reproduction of the bug that this fixes. Here's what's happening:

1. A Slate node n is rendered to a DOM element el with an onClick event that deletes n from the Slate document.
2. el is clicked, the onClick handler runs, and the node n is deleted.
3. The click event then propagates to this top-level Component.onClick handler, which wants to implement "if the user clicked on a void node, select it".
4. This Component.onClick wants to know whether we clicked on a void node, so it consults a reverse mapping from DOM elements back to Slate nodes, looking up the clicked DOM element el.
5. But there is no longer a valid Slate node corresponding to el, because its custom onClick handler has run already, which deleted its own Slate node (and in general, could have arbitrarily changed the Slate document).

This PR fixes the issue by adding some checks that the node is still at that path, before selecting that node.

This PR also adds an 'image delete' feature to the examples. My immediate motivation is to demonstrate the bug that this fixes. But this is also a very common editor feature, and I think it's valuable to show how to achieve it.

Issue
Fixes: #4240

Example

slate-delete-void.mp4

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: a7cbe9b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
slate-react	Patch

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[jameshfisher]

Note: I think an alternative fix could use Editor.pathRef. Create a reference to each void path at render time, then look up the pathRef in the onClick. But I think this would require some kind of scanning through the document at every render, to make a map of Path -> PathRef, which sounds complex and inefficient.

[jameshfisher]

(An alternative "fix" would be to remove the feature that selects void nodes when they're clicked. It's not clear to me that this is actually useful, and devs can always add this behavior themselves.)

[dylans]

I think this is the simpler option (compared to the other possible options you proposed), so let's go with this and see if it works sufficiently.