Add support for LX symlinks (WSL/MSYS2 native symlinks)

- Add reparseTagLxSymlink constant (0xA000001D)
- Implement decode logic for LX symlinks (UTF-8 format)
- Add IsLxSymlink field to ReparsePoint struct to preserve symlink type
- Implement encode logic to recreate LX symlinks on import
- Add unit tests for LX symlink round-trip validation

Fixes issue where Docker builds with MSYS2 failed with 'unsupported reparse point a000001d' error.

Signed-off-by: Varun Gokulnath <vagokuln@microsoft.com>
Signed-off-by: Varun Gokulnath <gvarun22@outlook.com>

reparse_lx_test.go

@@ -0,0 +1,60 @@
+//go:build windows
+// +build windows
+
+package winio
+
+import (
+	"testing"
+)
+
+func TestLxSymlinkRoundTrip(t *testing.T) {
+	// Test LX symlink encode/decode
+	original := &ReparsePoint{
+		Target:      "/usr/bin/bash",
+		IsMountPoint: false,
+		IsLxSymlink:  true,
+	}
+
+	// Encode
+	encoded := EncodeReparsePoint(original)
+	
+	// Decode
+	decoded, err := DecodeReparsePoint(encoded)
+	if err != nil {
+		t.Fatalf("Failed to decode: %v", err)
+	}
+
+	// Verify
+	if decoded.Target != original.Target {
+		t.Errorf("Target mismatch: got %q, want %q", decoded.Target, original.Target)
+	}
+	if decoded.IsLxSymlink != original.IsLxSymlink {
+		t.Errorf("IsLxSymlink mismatch: got %v, want %v", decoded.IsLxSymlink, original.IsLxSymlink)
+	}
+	if decoded.IsMountPoint != original.IsMountPoint {
+		t.Errorf("IsMountPoint mismatch: got %v, want %v", decoded.IsMountPoint, original.IsMountPoint)
+	}
+}
+
+func TestWindowsSymlinkNotLx(t *testing.T) {
+	// Test that regular Windows symlinks are not marked as LX
+	original := &ReparsePoint{
+		Target:      `C:\Windows\System32`,
+		IsMountPoint: false,
+		IsLxSymlink:  false,
+	}
+
+	// Encode
+	encoded := EncodeReparsePoint(original)
+	
+	// Decode
+	decoded, err := DecodeReparsePoint(encoded)
+	if err != nil {
+		t.Fatalf("Failed to decode: %v", err)
+	}
+
+	// Verify it's NOT an LX symlink
+	if decoded.IsLxSymlink {
+		t.Errorf("Windows symlink incorrectly marked as LX symlink")
+	}
+}