Add `@LazyProperty` to explicitly indicate a property delegated by `lazy`

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/utils/LazyProperty.kt

+/*
+ * Copyright 2020 Mamoe Technologies and contributors.
+ *
+ * 此源代码的使用受 GNU AFFERO GENERAL PUBLIC LICENSE version 3 许可证的约束, 可以在以下链接找到该许可证.
+ * Use of this source code is governed by the GNU AGPLv3 license that can be found through the following link.
+ *
+ * https://github.com/mamoe/mirai/blob/master/LICENSE
+ */
+
+package net.mamoe.mirai.utils
+
+/**
+ * 表示这个属性由 [lazy] 委托, 即它只会在被需要的时候才初始化.
+ */
+@Target(AnnotationTarget.PROPERTY)
+@Retention(AnnotationRetention.BINARY)
+annotation class LazyProperty
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