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`.
Co-authored-by: Cory Benfield <lukasa@apple.com>
Motivation:
In #3363 we converted `_NIOFileSystem` to `NIOFileSystem` and removed
the (unreleased) overloads for FilePath/NIOFilePath. This change adds
back `_NIOFileSystem` such that it matches the API it had at 2.86.0.
Modifications:
- Add back `_NIOFileSystem` and `_NIOFileSystemFoundationCompat` such
that their API is at 2.86.0
Result:
- `NIOFileSystem` uses `NIOFilePath`
- `_NIOFileSystem` uses `FilePath`
Karan Lokchandani
George Barnett