LibJS: Implement Function.prototype.bind()

Author: https://github.com/jack-karamanian
Commit: 1fa0c7304d
Pull-request: https://github.com/SerenityOS/serenity/pull/1863
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/linusg

Libraries/LibJS/Forward.h

@@ -52,6 +52,7 @@
 namespace JS {
 
 class ASTNode;
+class BoundFunction;
 class Cell;
 class DeferGC;
 class Error;