Add string escape characters to PowerShell syntax.
Summary: Powershell uses the syntax <char> or u{hex} as escape sequences within double-quoted strings. This adds highlighting support for both sequences to the PowerShell syntax.
Reviewers: dhaumann, cullmann
Reviewed By: dhaumann, cullmann
Subscribers: cullmann, dhaumann, kwrite-devel, kde-frameworks-devel
Tags: Kate, Frameworks
Differential Revision: https://phabricator.kde.org/D25054