[Feature Request] Warn when setting qmk_{firmware,userspace} paths if they contain spaces

[agsimmons]

Description

Due to make and possibly other build build tools used for qmk, a space in your paths to qmk_firmware or qmk_userspace can cause compilation to fail. Specifically, I had no issue with a space in my path until I started using External Userspace. While using External Userspace with a space in the path for both qmk_firmware and qmk_userspace, qmk compile and qmk userspace-compile fail with unhelpful errors.

While it may not be possibly to fully support spaces in paths, it might be a good idea to not allow spaces in paths to begin with. That could be enforced at the qmk_cli level. When user.qmk_home or user.overlay_dir are set, there could be a check for spaces, and an error if they are present.