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/assignment.wren

@@ -0,0 +1,18 @@
+var Nonlocal = "before"
+IO.print(Nonlocal) // expect: before
+Nonlocal = "after"
+IO.print(Nonlocal) // expect: after
+
+class Foo {
+  static method {
+    Nonlocal = "method"
+  }
+}
+
+Foo.method
+IO.print(Nonlocal) // expect: method
+
+new Fn {
+  Nonlocal = "fn"
+}.call
+IO.print(Nonlocal) // expect: fn