feat: expose True() and False() constructors

[cnaples79]

Summary

Expose two small convenience constructors in the public API:

• : equivalent to the empty schema, validates any instance (marshals as ).
• : equivalent to , validates no instance (marshals as ).

Rationale

When constructing schemas programmatically, it’s common to set fields (e.g. ) to JSON Schema booleans. Today this requires non-obvious patterns like . Public helpers improve readability and avoid duplicating these helpers across downstream codebases.

Changes

• Add and in .
• Tests: marshal behavior (/) and basic validate behavior using .

Fixes #13