Handle dynamic registration using different methods

Extremely annoyingly, some methods are dynamically registered under a different method. Notably the semantic tokens methods are registered under `textDocument/semanticTokens`, which is not actually a method that is defined.

This information is in the metamodel ("registrationMethod") so we _can_ do something with it. Probably we need to add a method like the custom method for each of them, and then have _another_ type family that maps methods to their registration methods, which will be the identity in most cases but not all.

Start by examining the metamodel's "registrationMethod" data and how semantic tokens methods are generated or registered. Trace the existing custom-method handling and determine how method-to-registration-method mapping should work; done means dynamically registered methods use their declared registration method, with identity mapping for methods without an override.

Written by the indexing model from the issue text.