Add util readResource

mirai-core-utils/src/commonMain/kotlin/Resources.kt

@@ -0,0 +1,16 @@
+/*
+ * Copyright 2019-2021 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/dev/LICENSE
+ */
+
+package net.mamoe.mirai.utils
+
+
+@TestOnly
+public fun readResource(url: String): String =
+    Thread.currentThread().contextClassLoader.getResourceAsStream(url)?.readBytes()?.decodeToString()
+        ?: error("Could not find resource '$url'")