VSCode support

New features:

• Add VSCode support and documentation, see README.md and doc/vscode.md for details.
  • Working C++ and Python intellisense support!
• Replace Project.xenv by Project-build.xenv and patch Gaudi's python packages to be intellisense friendly
  • For example, changes in the python in Rec are immediately available when running Moore (no need to make).

Improvements:

• Instructions how to increase build verbosity, closes #25 (closed).
• Copy compile commands and runtime env to .output in order to preserve them when purging (keeps intellisense happy), closes #28 (closed).

Internal changes:

• Switch to Python 3 everywhere.