add syntax highlight for Vim

[mattn]

[alandonovan]

Hi Matt, thanks for contributing this change. I'm not sure yet where this feature belongs as it doesn't relate specifically to the Go implementation of Skylark, but various possible alternative locations are equally unnecessarily tied to the Java implementation. It seems we need a place for implementation-language-agnostic helper programs like this one. I'm discussing it with the Bazel team and will let you know what we conclude.

More specifically, I wonder whether this change is necessary at all. Syntactically, Skylark is a strict subset of Python, so I would expect Skylark to be well supported by all existing editors if they simply treat it as Python.