Add homeDirectory accessor to FileSystem

[PhantomInTheWire]

Add homeDirectory accessor to FileSystem

Motivation:

Addresses #3381 by adding a homeDirectory property to FileSystem, equivalent to FileManager.default.homeDirectoryForCurrentUser.

Modifications:

• Added homeDirectory property to FileSystemProtocol and implemented in FileSystem
• Implementation checks HOME environment variable first, falls back to USERPROFILE on Windows, or uses getpwuid_r(3) on POSIX systems
• Added system call wrappers (system_getuid, libc_getpwuid_r) with proper platform guards
• Added FileSystemError.getpwuid_r() error helper

Result:

Users can now access the home directory via FileSystem.homeDirectory, returning a FilePath asynchronously. Follows the same pattern as currentWorkingDirectory and temporaryDirectory.

[PhantomInTheWire]

rebased to master. @Lukasa shoudl i be worried about this in ci?

1 breaking change detected in _NIOFileSystem:
💔 API breakage: var FileSystemProtocol.homeDirectory has been added as a protocol requirement
Error: Process completed with exit code 1.

[Lukasa]

No, this is acceptable.

[PhantomInTheWire]

ci failure seems unrelated

[PhantomInTheWire]

@Lukasa just to confirm this will get auto merged the next time swift nio has a semver minor or major right? (sorry if this is too basic of question to ask, but im just confused)

[Lukasa]

API breakage is expected, merging over it.