Describe wasi-filesystem's path sandboxing behavior

[sunfishcode]

Add a new path-resolution.md document describing CloudABI-style path resolution behavior. wit/types.wit previously had a brief description of this behavior, however this new document presents it in much more detail, with a description of the sandboxing properties and a discussion of implementation techniques.

[sunfishcode]

I've answered all the questions above; I'm going to go ahead and merge this now. If there are any further questions; please file new issues!