Lexer for Inform 7

This pull request adds a lexer for Inform 7 as requested in #1725.
The lexer is at: https://github.com/PogiNate/Sublime-Inform.

I opened an issue on the repository to clarify the license situation: [PogiNate/Sublime-Inform#2](https://github.com/PogiNate/Sublime-Inform/issues/2).

I think we'll need to say `source.Inform7` here to match the case used in Sublime-Inform itself.

Running `script/prune-grammars` should also remove the old inform grammar (after verifying that it's no longer used).