Add v2 Rego version

[johanfylling]

For opting-in to new Rego v2 syntax and semantics.

• Add RegoV2 language enum
• Add rego_v2 capability feature
• Add rego.v2 import

Don't add rego_v2 feature to default set of enabled capabilities until first user-facing Rego V2 feature has been added.

[johanfylling]

A bit premature to do this now; let's wait until we have a mature v2 language profile.