Support conditional arguments in Sorbet compiler

[Mangara]

Fixes #591

When generating types for build_enumerator definitions with both required positional arguments and other types of arguments:

def build_enumerator(user_id, foo_id = nil, cursor:)

The compiler failed with:

/Users/sander/.gem/ruby/3.3.5/gems/job-iteration-1.10.0/lib/tapioca/dsl/compilers/job_iteration.rb:60:in `sort_by': ArgumentError: comparison of NilClass with 1 failed (Parallel::UndumpableException)
	from /Users/sander/.gem/ruby/3.3.5/gems/job-iteration-1.10.0/lib/tapioca/dsl/compilers/job_iteration.rb:60:in `sort_by!'
	from /Users/sander/.gem/ruby/3.3.5/gems/job-iteration-1.10.0/lib/tapioca/dsl/compilers/job_iteration.rb:60:in `block in decorate'
	from /Users/sander/.gem/ruby/3.3.5/gems/tapioca-0.16.5/lib/tapioca/rbi_ext/model.rb:40:in `block in create_class'
	from <internal:kernel>:90:in `tap'
	from /Users/sander/.gem/ruby/3.3.5/gems/tapioca-0.16.5/lib/tapioca/rbi_ext/model.rb:39:in `create_class'
	from /Users/sander/.gem/ruby/3.3.5/gems/tapioca-0.16.5/lib/tapioca/rbi_ext/model.rb:16:in `create_path'
	from /Users/sander/.gem/ruby/3.3.5/gems/job-iteration-1.10.0/lib/tapioca/dsl/compilers/job_iteration.rb:27:in `decorate'
	from /Users/sander/.gem/ruby/3.3.5/gems/sorbet-runtime-0.5.11834/lib/types/private/methods/_methods.rb:279:in `bind_call'
	from /Users/sander/.gem/ruby/3.3.5/gems/sorbet-runtime-0.5.11834/lib/types/private/methods/_methods.rb:279:in `block in _on_method_added'

Looking at the parameter types and indices, this becomes clear:

types: [RBI::ReqParam, RBI::OptParam]
indexes: [nil, 1]

Because RBI::ReqParam is not in PARAM_TYPES_IN_ORDER, its index in that list is nil, and sorting it in a list with integers doesn't work. (The existing tests used either all mandatory positional arguments, or no mandatory positional arguments, so they didn't hit this case, and sorting a list with all nil elements does work.)

It turns out that RBI::Param is the superclass, and doesn't tend to occur as the concrete parameter type, so we can just replace it with RBI::ReqParam to fix it.

I also added some more test cases with optional parameters.