[Feature Request] Create artifact files in a separate build directory

[zapta]

Currently artifact files that commands such as apio build or apio test create are placed in the source directory which has
a few disadvantages, such as the difficulty to capture all of them with a .gitignore file and the general mixup between source and artifact files.

This request is for generating all of the artifact files in a subdirectory, e.g. '_build', similar to the platformio's .build file.

Scons has a feature called VariantDir() that may help achieving that but I haven't look into it yet.