Implement schema statements autocompletion in Active Record migrations

[janko]

Active Record migrations define method_missing that delegates any unknown methods to the underlying connection adapter. That means that implementing autocompletion of schema statements is mostly achieved by completing connection adapter methods.

Because Active Record migrations inherit from ActiveRecord::Migration[<version>], which is not a direct class reference, I couldn't use NodeContext#nesting, as that returns strings. I had to inspect the nesting nodes in Prism, which is available via the @nesting_nodes variable. Maybe we could define a public NodeContext#nesting_nodes attribute reader?

I implemented a server call that returns database configurations, so that I can map the migration directory to the corresponding connection adapter, in case multiple database adapters are used.

ActiveRecord::Migration implements a few additional methods (e.g. reversible), and it would be nice to have TableDefinition/AlterTable completion in create_table/change_table blocks at some point, but I wanted to first tackle the primary functionality.

Closes #444