Remove -Focus from the public API

[nohwnd]

As mentioned in #721 the -Focus functionality is potentially harmful for building on build servers and if integrated well with VSCode it can be replaced with UI integration almost entirely. Remove this from the public API for the initial release.

[nohwnd]

I still find this immensly useful, especially because I can set the focus on whole block and just re-run without hunting for the correct 'Run tests' button to press...

[nohwnd]

Removed in #1451