Nested repositories don't work

[Talon1024]

While I was trying to "fix" the syntax highlighting for heredocs in PHP, I noticed that if I moved the heredoc_interior rule out of the nested repository in the heredoc rule, the heredocs would be tokenized correctly.

However, since the heredoc_interior rule is in a nested repository in the original grammar, it is not recognized by first-mate.

I have begun work on this in my own fork of first-mate.