Support environment variables in Node.js

[lppedd]

I've applied the following changes:

• Deleted ANTLRFileStream: it was deprecated, + let's leave the responsibility of reading files to consumers of the library.
  It's better for us, and generally will result in better code for them.
• Deleted Platform (Platform.readFile): again, we don't have to read files anymore
• Cleaned up deprecations: removed almost all deprecated methods.
  This ensures we don't end up supporting stuff that shouldn't have been there in the first place.
• Cleanup up - again - the runtime code a little bit
• Made Lexer.tokenNames elements non-null at the type level

[ftomassetti]

Looks good to me, I have just minor comments

[ftomassetti]

Thank you for the PR, I am merging it