feat: export FormResult and related types for better type inference

[gabrielseco]

Problem

When using createHeadlessForm() with async hooks like useQuery, TypeScript couldn't infer the return type, resulting in unknown type:

const { data } = useQuery({  queryFn: async () => createHeadlessForm(schema),})// ❌ data is typed as 'unknown' - no intellisense or type safety

Solution

Export the core types from the public API so TypeScript can properly infer them:
const { data } = useQuery({ queryFn: async () => createHeadlessForm(schema),})// ✅ data is now properly typed

Tested in local with the https://github.com/remoteoss/remote-flows package to make sure that it worked