Changes:
- Add missing lazy assignment for OS_NAME, fixes double Nix shells
- Moved `scripts/add-nix-gcroots.sh` to `nix/scripts/gcroots.sh`
- Moved Nix package manager setup to `nix/scripts/setup.sh`
- Created `nix/scripts/source.sh` to reuse in all Nix scripts
- Created `STARTING_GUIDE.md` with instructions for contributors
- Created `scripts/colors.sh` for definition of shell colors
- Removed `scripts/setup` in favor of `nix/scripts/setup.sh`
- Removed all of `scripts/lib` since it was useless
Signed-off-by: Jakub Sokołowski <jakub@status.im>
