LibJS: Perform function instantiation in bytecode

This replaces Bytecode::Op::EnterScope with a new NewFunction op that
instantiates a ScriptFunction from a given FunctionNode (AST).

This is then used to instantiate the local functions directly from
bytecode when entering a ScopeNode. :^)

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -39,7 +39,7 @@
     O(JumpConditional)            \
     O(JumpNullish)                \
     O(Call)                       \
-    O(EnterScope)                 \
+    O(NewFunction)                \
     O(Return)                     \
     O(BitwiseAnd)                 \
     O(BitwiseOr)                  \