Recognize non-letter Windows drive designators

[aryansk]

Fixes #271

What changed

• Recognize any single non-separator, non-colon character as a Windows drive designator instead of restricting the syntax to ASCII letters.
• Add simulated-Windows coverage for +:\\foo; keep the Unicode €: case gated to native Windows where the platform character type supports it.

Validation

• swift test --filter FilePathSyntaxTest — 6 tests passed.

[glessard]

FilePath is being moved to the standard library (SE-0529). Consider contributing there once the change has landed.

[aryansk]

Thanks for the heads-up. I’ll revisit the drive-designator behavior in the standard-library FilePath implementation once SE-0529 lands; this draft remains scoped to the current swift-system implementation.