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/closure/close_over_method_parameter.wren

@@ -1,12 +1,12 @@
-var f = null
+var F = null
 
 class Foo {
   method(param) {
-    f = new Fn {
+    F = new Fn {
       IO.print(param)
     }
   }
 }
 
 (new Foo).method("param")
-f.call // expect: param
+F.call // expect: param

test/closure/use_global_in_function.wren

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

test/closure/use_global_in_method.wren

@@ -1,15 +0,0 @@
-var global = "global"
-// TODO: Forward reference to global declared after use.
-
-class Foo {
-  method {
-    IO.print(global)
-  }
-
-  static classMethod {
-    IO.print(global)
-  }
-}
-
-(new Foo).method // expect: global
-Foo.classMethod // expect: global