Disable custom field types

[JedWatson]

We've been discussing whether having the ability to create custom field types in Keystatic is worth continuing with, because it complicates the internal API surface area (and what we need to consider when making changes, in terms of trying not to break public API) quite a bit, slowing down core development somewhat.

I'm not sure if this feature is widely used or especially valuable, but have just discovered though #453 that it's linked to bundle size increases when using the Reader API as well, which is strong motivation to make some changes.

So with this issue, I propose we lock down the field types to just the built-in ones for now, and reconsider what/when we need to do to open up more customisation down the track.

If anybody has created custom field types and would be adversely affected by this change, please shout out here or create a discussion with details about your use-case so we can take that into account.

And in the meantime I expect we'll ship an update to fix #453 that disables the custom field API with a warning linking to this issue.

Thanks!