RFC: Split FilePath constructor to guarantee constructor with AmbientAuth

[Theodus]

It should be without error to construct a FilePath when AmbientAuth is available as FilePath is a pairing between some path string and capabilities on that path. For capabilities, the current constructor uses a union of (FilePath | AmbientAuth). Insufficient capabilities or access allows the first half of this union, FilePath, to fail, however the latter half, AmbientAuth, can never fail.

https://github.com/ponylang/rfcs/blob/main/text/0070-filepath-constructor.md