Make JSValue's subscript setter nonmutating

Close #132

Co-authored-by: Casper Zandbergen <info@casperzandbergen.nl>

Author: kateinoigakukun

Sources/JavaScriptKit/JSValue.swift

@@ -120,14 +120,14 @@ extension JSValue {
     /// - Precondition: `self` must be a JavaScript Object.
     public subscript(dynamicMember name: String) -> JSValue {
         get { self.object![name] }
-        set { self.object![name] = newValue }
+        nonmutating set { self.object![name] = newValue }
     }
 
     /// An unsafe convenience method of `JSObject.subscript(_ index: Int) -> JSValue`
     /// - Precondition: `self` must be a JavaScript Object.
     public subscript(_ index: Int) -> JSValue {
         get { object![index] }
-        set { object![index] = newValue }
+        nonmutating set { object![index] = newValue }
     }
 }