Add powershell builtins

[miparnisari]

Fixes #966

How it looks (august 11)

[dblessing]

Thanks @miparnisari

Something doesn't seem quite right here. When I run the sample the latter part doesn't seem to be getting highlighted correctly?

[markroloff]

On line 621, looks like the foreach keyword was removed. You'll want to keep that in there as it's both an alias and a built-in.

[miparnisari]

@dblessing that's because builtins are colored white :)

@markroloff oh, then I should make the regexes case sensitive because otherwise the lexer won't distinguish between foreach and ForEach.

[dblessing]

@miparnisari Interesting. It doesn't look great from a highlighting perspective, but that's something we can/should address in another issue/PR.

Looks like some things changed, though. See this side-by-side comparison of before (left) and after (right). Most notably, try and catch are highlighted differently but also less apparent are several other things.