Make all types support "!" so "if (!foo)" works reliably for all objects.

test/null/literal.wren

@@ -0,0 +1 @@
+IO.print(null) // expect: null

test/null/not.wren

@@ -0,0 +1 @@
+IO.print(!null) // expect: true

test/null/type.wren

@@ -0,0 +1,4 @@
+IO.print(null is Null)      // expect: true
+IO.print(null is Object)    // expect: true
+IO.print(null is Bool)      // expect: false
+IO.print(null.type == Null) // expect: true