Make Configuration introspectable

[broken-circle]

Closes #206.

This PR adds public introspection of Arguments, Executable, and Environment so callers generating subprocess configurations can unit-test them without spawning a subprocess. The current storage of these types is opaque, leaving callers with only string-based descriptions to parse.

• Arguments conforms to RandomAccessCollection<Arguments.Value>, where Value is a public mirror of the internal StringOrRawBytes. A Value enum was chosen over lossy UTF-8 decoding so callers constructing arguments from [UInt8] on POSIX can verify the exact bytes.

• Arguments.executablePathOverride is exposed as a separate public property rather than folded into iteration. Folding would make argv[0] ambiguous between an override and a regular first argument, and would prevent callers from distinguishing the two cases in tests. The internal stored property is renamed to _executablePathOverride to free the name for the public accessor.

• Executable and Environment expose their contents through new Representation mirror enums rather than exposing their internal storage types directly, decoupling the public introspection surface from internal storage. Environment.Key.path is promoted from package to public so callers can use it as a canonical reference to the PATH key.

The Swift tools version is bumped to 6.2.3 to support @nonexhaustive (SE-0487).

[jakepetroules]

The capability is nice, but I'm a little concerned about the use of enums in public API -- usually that's discouraged. I'm not sure what other good options there are, though.

[allevato]

Making it a @nonexhaustive enum would avoid the issues that tended to cause problems with evolution for enum-based APIs, I think?

[broken-circle]

Nice, I was just wondering if something like @nonexhaustive existed. Reading SE-0487, it seems appropriate here.

We could annotate Arguments.Value and Executable.Representation with @frozen, since their cases are exhaustive. Environment.Representation can be @nonexhaustive.

[iCharlesHu]

@broken-circle thanks for the PR! Let's do this post 1.0 (it was originally scheduled for post 1.0)

[broken-circle]

This is ready for review now that we're post-1.0.

Earlier I thought Package.swift was fine as-is, but the tools version needed bumping to 6.2.3, since that's when @nonexhaustive was implemented. The CMake build doesn't declare a Swift floor; should we add a CMAKE_Swift_COMPILER_VERSION guard?

If the version bump isn't acceptable, we can make Environment.Representation @frozen instead, although this enum's set of members isn't necessarily closed. For example, there's currently no way to inherit the parent environment and override a value with raw bytes, and filling that gap would need a new case, and therefore a new major version.