Add enum for reporter scope

Some reporters are sprite-specific, such as the costume name/number or
X position of the sprite. Others are global. This enum allows an
extension author to specify the scope of a reporter.

src/extension-support/reporter-scope.js

@@ -0,0 +1,6 @@
+const ReporterScope = {
+    GLOBAL: 'global',
+    SPRITE: 'sprite'
+};
+
+module.exports = ReporterScope;