feat: add `deriving noncomputable instance` syntax

This PR add `deriving noncomputable instance Foo for Bar` syntax so that
delta-derived instances can be marked noncomputable. When the underlying
instance is noncomputable, `deriving instance` now gives an actionable
error with a "Try this:" suggestion pointing to the noncomputable variant.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>