Powershell mode for CodeMirror

Now it actually works!

What it has:

1. All standard PowerShell syntax
2. Highlighting of built-in functions and variables (e.g. Test-Path or Write-Warning)
3. Highlighting of string interpolation in single-line strings

What it does not have:

1. Highlighting of string interpolation in here-strings
2. Any special handling of attributes (e.g. [switch] would be highlighted as a keyword in braces)