Write SpiderMonkey 68 migration tool

The main thing that SpiderMonkey 68 (#270 (closed)) is going to remove, is String generics. Most uses of these should be easy to catch via Reflect.parse.

We don't need to wait for GJS to be ported, in order to already create a migration tool similar to https://gitlab.gnome.org/ptomato/moz60tool that GNOME apps can run on their JS code. Such a tool could detect CallExpressions where the callee is a MemberExpression consisting of String and one of the removed generic methods.