Add support for alias $x $y

[sampersand]

One of the lesser-known facts of ruby is you can also use alias to alias global variables to one another, such as alias $LOAD_PATH $:.

I propose we support this syntax in RBS (it'd be useful for require 'English'). It's very, very niche, but should be supported eventually for completeness

[sampersand]

Moved to an issue: #1647