Live Preview message events were typed with the generic `MessageEvent`
interface without passing any of the Live Preview specific properties,
leading to unknown types upon use. To fix this, there is a new
`LivePreviewMessageEvent` which properly extends the underlying
`MessageEvent` interface, providing much needed type safety to these
functions. In the same vein, the `UpdatedDocument` type was not being
properly shared across packages, leading to multiple independent
definitions of this type. This type is now exported from `payload`
itself and renamed to `DocumentEvent` for improved semantics. Same with
the `FieldSchemaJSON` type. This PR also adjusts where globally scoped
variables are set, putting them within the shared `_payloadLivePreview`
namespace instead of setting them individually at the top-level.
466f109152