Add a new BuildExtensions package that contains a UserPreference macro.

The build extensions package can be used for both macros and plugins.

BuildExtensions/Package.swift

@@ -0,0 +1,43 @@
+// swift-tools-version: 6.2
+
+import CompilerPluginSupport
+import PackageDescription
+
+let package = Package(
+    name: "BuildExtensions",
+    platforms: [.macOS(.v15)],
+    products: [
+        .library(name: "Macros", targets: ["Macros"])
+    ],
+    dependencies: [
+        .package(url: "https://github.com/swiftlang/swift-syntax", from: "602.0.0")
+    ],
+    targets: [
+        // The public macro declarations imported by the app.
+        .target(name: "Macros",
+                dependencies: ["MacrosImplementation"],
+                swiftSettings: [
+                    .enableUpcomingFeature("NonisolatedNonsendingByDefault"),
+                    .enableUpcomingFeature("InferIsolatedConformances")
+                ]),
+        // The compiler plugin holding the SwiftSyntax expansions.
+        .macro(name: "MacrosImplementation",
+               dependencies: [
+                   .product(name: "SwiftSyntaxMacros", package: "swift-syntax"),
+                   .product(name: "SwiftCompilerPlugin", package: "swift-syntax")
+               ],
+               swiftSettings: [
+                    .enableUpcomingFeature("NonisolatedNonsendingByDefault"),
+                    .enableUpcomingFeature("InferIsolatedConformances")
+               ]),
+        .testTarget(name: "MacrosTests",
+                    dependencies: [
+                        "MacrosImplementation",
+                        .product(name: "SwiftSyntaxMacrosTestSupport", package: "swift-syntax")
+                    ],
+                    swiftSettings: [
+                        .enableUpcomingFeature("NonisolatedNonsendingByDefault"),
+                        .enableUpcomingFeature("InferIsolatedConformances")
+                    ])
+    ]
+)