Proposal: Support auto extending module

[tk0miya]

In Ruby language, developers need to use both "include" and "extend" to implement mix-in having instance methods and class methods. The auto extending module is a well-known technique to realize it (ex. ActiveSupport::Concern, "extend" call in included block, and so on).

This supports the auto extending modules via modules having "autoextend:..." annotation. The RBS::DefinitionBuilder searches the extended modules from the annotations of the included modules.