Add a new BuildExtensions package that contains a UserPreference macro.

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

BuildExtensions/Sources/Macros/UserPreference.swift

@@ -0,0 +1,29 @@
+//
+// Copyright 2026 Element Creations Ltd.
+//
+// SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+// Please see LICENSE files in the repository root for full details.
+//
+
+/// Exposes a value backed by keyed storage as a computed property, alongside a `<name>Publisher`
+/// peer that emits the current value and every subsequent change.
+///
+/// The generated accessors read and write through the enclosing type's `store` (a
+/// `UserDefaultsProtocol`); a `volatile` preference is instead held in memory and resets on launch.
+///
+/// When `key` is omitted the property's own name is used as the storage key. Provide an explicit
+/// `key` whenever the storage key must differ from the property name (e.g. to keep matching a value
+/// persisted under a legacy key).
+///
+/// A `reset<Name>()` method is also generated, which clears the stored value so it reverts to the
+/// default.
+@attached(accessor)
+@attached(peer, names: arbitrary)
+public macro UserPreference<Value>(key: String? = nil, defaultValue: Value, volatile: Bool = false) =
+    #externalMacro(module: "MacrosImplementation", type: "UserPreferenceMacro")
+
+/// A variant for optional preferences that default to `nil`.
+@attached(accessor)
+@attached(peer, names: arbitrary)
+public macro UserPreference(key: String? = nil, volatile: Bool = false) =
+    #externalMacro(module: "MacrosImplementation", type: "UserPreferenceMacro")