LibJS: Implement a very hackish "arguments" object

We now lazily create an "arguments" array inside functions when code
tries to access it.

This doesn't follow the spec at all but still covers a lot of the
basic uses of arguments, i.e "arguments.length" and "arguments[n]"

Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -62,6 +62,7 @@ namespace JS {
     P(abs)                                   \
     P(acosh)                                 \
     P(apply)                                 \
+    P(arguments)                             \
     P(asIntN)                                \
     P(asUintN)                               \
     P(asinh)                                 \