Description of changes

Creates a new lib.path library component, originally proposed in #200718. This commit adds two main parts of it:

• The design document covering the main design decisions for this library
• A lib.path.subpath.isValid function to check whether a value is a valid subpath according to the design document
• A lib.path.subpath.normalise function implementing a safe normalisation of subpaths according to the design document.

In the future further library functions will be implemented upon these two functions, it's the working horse of this path library. The next functions from #200718 that can be implemented are subpath.join and append, see #210426 for progress.

This work is sponsored by Antithesis ✨

Things done

• Wrote documentation
• Wrote code-covering tests
• Wrote property tests