Improve TryWhy3

1. File examples/config.json is no longer used for settings about step limits. They are now directly recovered from trywhy3.html. That way, user settings in the local storage are no longer lost.
2. Settings with default values are no longer stored in the local storage, so that upstream default settings take precedence.
3. File examples/config.json is now used for examples, in a per-language way. This removes the need for ad-hoc index files.
4. Output of execution (e.g., use of io.StdIO) is now captured and displayed.
5. There is no longer a dummy format in the format list.
6. Numerous cleanups.