Add ENV signature

[ksss]

ENV is an instance of Object class.
But, RBS does not have syntax for extended object.
Therefore, I introduce the temporary defined class ENVClass.

Options not selected

interface _ENV

interface _ENV
end
::ENV: _ENV

😁 No need to introduce a new class.
😥 interface does not include Enumerable.

singleton methods

class ENV
  def self.[]: (String) -> String?
end

😁 No need to introduce a new class.
😥 It's not correct and does not include Enumerable.

Special implementation

Special implementation in external tool (steep) for ENV.

😁 No need to introduce a new class.
😥 Eventually, we'll need a type definition.

Add syntax for extended object to RBS

(sample)

def ENV.[]: (String) -> String?

😁 Can be expressed correctly.
😥 Just for the ENV?

[soutaro]

Hmm. Thank you for reporting a really interesting problem. I don't know what to do for ENV...

Merging this PR for now. 🙏