where {T} doesn't highlight T as type

[davidanthoff]

First reported by @cossio in julia-vscode/julia-vscode#396.

I'm opening an issue here because we are actually just copying the grammar file from this repo here periodically over to the VS Code extension, so it would probably make more sense to fix this here first and then we'll just get it automatically at some point.

Also, I don't understand the grammar files, so hoping that someone else could fix this ;)

[pfitzseb]

This can maybe be handled in a manner similar to what I did in #130.