Support linting `rq` files

[anderseknert]

Regal should be able to lint rq files "out of the box", meaning that if:

• A user runs regal lint script.rq, we should recognize it as Rego with rq additions.
• A user runs regal lint script we should recognize it as Rego if the file contents looks like Rego.
• A user runs regal lint my_dir/ we should be able to recognize rq files pointed out in config.

I've probably missed some things as I've still more to learn about rq. Note that the scope of this issue is intentionally limited to "support". Once that's in place we can consider rq-specific linter rules.

Related:

• Support capabilities feature from OPA
• Allow configuration to decide what files to parse

Tagging @charlesdaniels as a stakeholder :)