Resolve rbs type aliases and interfaces from their definitions

[tompng]

Fixes the nightly CI failure of test-rbs-versions (latest): rbs HEAD now uses builtin type aliases (range[int], array[U], etc.) in core method signatures where concrete classes and interfaces used to appear (failure example).

Introduce InterfaceType for rbs interface and type alias resolution

• Type aliases are now expanded from their definitions with DefinitionBuilder#expand_alias2 (available since rbs 2.7.0) instead of mapping known alias names to hardcoded types, so future alias migrations in rbs core signatures keep working, and user-defined aliases resolve too.
• rbs interfaces become Types::InterfaceType instead of falling back to Object. Completion candidates come from the interface definition, type variables are bound through named_params (_ToAry[Integer]#to_ary → Array[Integer]), and overload scoring matches arguments by duck typing ([1][0.5]. completes as Integer because Float#to_int satisfies _ToInt), generalizing the previous ad-hoc to_str/to_int/to_ary checks.
• Also fixes RBS::Types::Bases::Class conversion which discarded the transform result and referenced an undefined variable.

Limit type alias and interface expansion depth

Recursive aliases guarded by a type constructor (e.g. type json = Integer | Array[json]) are valid RBS — rbs validate only rejects unguarded recursion, and that check does not run on environment load. Eager expansion caused SystemStackError on recursive aliases, and alias chains like type c = b | [b]; type b = a | [a] can expand exponentially even without recursion (which a visited set would not bound). Expansion nesting is limited to 3, falling back to Object at the cutoff, with the count threaded as an argument to stay stateless. The limit also covers the interface case of _match_free_variable, which could recurse infinitely on cyclic generic interfaces.

Testing

• bundle exec rake test passes with rbs 2.7.0, 3.0, 3.3, 3.4, 3.6, 3.8, 3.10, 4.0 and HEAD (108 tests)
• Completion on a method whose rbs return type is a recursive json alias: previously SystemStackError (no candidates), now resolves in ~0.4ms including element access like info["a"][0].

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