Detect and support pre-configured build directories

[Flamefire]

Brief Issue Summary

Allow Usage of a pre-configured build directory.

Expected:

1. Configure project via cmake/ccmake/cmake-gui
2. Open project source folder in VSC
3. (optional) Set the build directory in the configuration if non-default
4. CMake Tools picks up the CMakeCache.txt and determines the used variant and Kit or shows something useful if this is not possible automatically
5. Intellisense/Include directories etc. work (requires setting the compilation database and possibly a reconfigure with the CMAKE_EXPORT_COMPILE_COMMANDS define)

Apparent Behavior:

• No sign that the CMakeCache is deteced
• Choosing a Kit matching the one used manually deletes the preconfigured build dir (see Is the cache being cleared on the first configure? #392)

[vector-of-bool]

This is one that people have asked for in the past.