chore: use ext property instead of func

src/main/kotlin/cn/rtast/fancybot/util/str/StringFormatter.kt

@@ -47,6 +47,4 @@ fun Int.formatToMinutes(): String {
     return "$minutes:${if (remainingSeconds < 10) "0" else ""}$remainingSeconds"
 }
 
-fun String.proxy(): String {
-    return this.replace("https://", "https://proxy.rtast.cn/https/")
-}
+val String.proxy: String get() = this.replace("https://", "https://proxy.rtast.cn/https/")