fix: allow adding new field types via overrides

[jaredhill4]

Closes #729.

With this change, developers can now configure their own custom field types like this:

fieldTypes: {
  image: ({ onChange }) => <input type="file" onChange={onChange} />,
},

I realize there are likely some type considerations, but it worked for me.

---

Demo

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[chrisvxd]

This seems... too simple. I'll sanity check before merging.

[jaredhill4]

This seems... too simple. I'll sanity check before merging.

Yea there were a couple considerations:

1. UsingdefaultFields for children was what throwing the error when trying to use a custom field type in overrides.
2. The overrides.fieldTypes is spread into render, so we should just use render from there down (unless the goal or thought was to error when trying to use custom field types).
3. From what I could tell, passing children to Render seemed unnecessary.

[chrisvxd]

I've done some more thinking on this, and it is indeed too good to be true.

You're removing children because it's getting in the way of the user field type specified in overrides, but the children is necessary as overrides expect the default node to be provided to the override as the children prop, which enables the user to wrap the node.

The correct fix for runtime behaviour would be to conditionally check the render function, or default to null (const children = defaultFields[field.type]?.(mergedProps) ?? null;).

However, the type issues are significant and require much further thought before any solution can be merged.

I'm going to close this one out for now whilst I think about the type issues.

Thanks!

[chrisvxd]

New attempt at fix: #1147