Add constant type for rbs

[shoma07]

In this PR, add type definition(rbs) to constant using @return.
If there is no @return, it continue to be untyped.

We can also write @return in constant, but I'm not sure if that's the right thing to do.
But I couldn't think of any other way to add the type.

example:

• ruby

  class A
    # @return [String]
    EXAMPLE_CONSTANT = 'Bar'
  end

• rbs

  class A
    EXAMPLE_CONSTANT: String
  end

I haven't changed rbi, so I couldn't determine if #128 could be closed.

[AaronC81]

Awesome - thanks for your contribution! Looks good to me.

Yeah, I think we'll leave #128 open for now until RBI works too.

[shoma07]

Thank you!