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/variable/outside_method.wren

@@ -0,0 +1,18 @@
+var foo = "variable"
+
+class Foo {
+  foo { "method" }
+
+  method {
+    IO.print(foo)
+  }
+
+  static foo { "class method" }
+
+  static classMethod {
+    IO.print(foo)
+  }
+}
+
+(new Foo).method // expect: method
+Foo.classMethod  // expect: class method