Allow user-provided LSP policies

[Anders Eknert]

Just like how Regal allows users to provide their own custom linter policies, we should allow them to so for Rego-based LSP features, like code completion. I don't think many are going to do that, but one of the benefits of us using Rego for this purpose is that custom additions can be provided at runtime, and that's an advantage we should leverage. Even if no one would use this, we'd still benefit in a few ways:

• We'll need to treat this as a public API / contract
• We'll need to document it as a public resource
• We'd be able to develop LSP features ourselves without having to rebuild the Regal bundle after every change

Just an idea right now, and we're not yet at a point where these APIs are stable enough to do this. But when we are, let's discuss this.