Add more highlighting support and stylesheet.

The stylesheet changes are non-functional essentially they introduce a default stylesheet for Powershell based on the popular "Son-Of-Obsidian: theme for Visual Studio. These commits can be ignored if required.

The other changes in this pull request include:
* Adding more comparison operators, `-gt`,`-lt` and explicitly case-insensitive `-ieq` and case-sensitive `-ceq` equals operators.
* Adding support for highlighting the `$` as part of the variable name which didn't appear to work previously.
* Adding support for highlighting variables within double-quoted strings (where the variable will be expanded).
* Adding support for highlighting the `#` at the beginning of single line comments separately from the comment text.