Add support for "nonlocal" (capitalized) names, and change how variable lookup works.

Inside a method, all local variable lookup stops at the method boundary. In other
words, methods, do not close over outer local variables.

If a name is not found inside a method and is lowercase, it's a method on this.
If it's capitalized, it's a global variable.

test/nonlocal/use_global_in_function.wren

@@ -0,0 +1,6 @@
+var Global = "global"
+// TODO: Forward reference to global declared after use.
+
+new Fn {
+  IO.print(Global) // expect: global
+}.call