FilePath is Decodable, but not from a String

it looks like [`FilePath`](https://github.com/apple/swift-system/blob/1d5e35a460ebef822d61181b42a7354abaeb80c3/Sources/System/FilePath/FilePath.swift#L70) has a conformance to `Codable`, but this is a synthesized conformance, and expects a keyed container with a property called `_storage`.

i don’t know if this is intentional, but it doesn’t seem very useful. `FilePath` should just be `Decodable` from a `String`.

Start in Sources/System/FilePath/FilePath.swift at the FilePath declaration and its Codable conformance. Check how the synthesized conformance handles storage and whether the project has tests covering Codable behavior. Done means FilePath can be decoded from a String while preserving the intended existing encoding behavior, with tests demonstrating the result.

Written by the indexing model from the issue text.