Add type arguments support to singleton types

[allcre]

ticket: https://github.com/Shopify/team-ruby-dx/issues/1460

Add support for parameterized singleton types

This PR adds support for type parameters on singleton types to match the functionality available in Sorbet's T.class_of(X)[Y] syntax. With this change, RBS now supports the equivalent syntax: singleton(X)[Y].

Changes

• Updated the RBS grammar to allow type arguments for singleton types
• Modified the C parser implementation to accept type parameters for singleton types

Examples

Previously, only this was supported:

singleton(Array)   # Class singleton type with no type parameters

Now this is also supported:

singleton(Array)[String]   # Class singleton type with String type parameter

Questions

1. Should the Application module be included in the ClassSingleton class, similar to how it's included in ClassInstance and Interface? Currently, I've implemented the necessary methods directly.

2. Are there additional methods such as map_type and each_type that should be added to the ClassSingleton class?