Add `rego.v2` directive `import`

Add the rego.v2 Rego import alongside the existing rego.v1. Importing it in a module applies the Rego v2 language definition to the active module (modules within the same compile set without the import are unaffected).

The Rego v2 language definition contains:

• import future.keywords.and
• import future.keywords.or
• import future.keywords.not

and nothing else; no additional semantics or strictness checks.