SlimerJS script for automated screenshot/thumbnail generation for examples

- as discussed in #3871
- added to /utils/build/
- generate all examples to /examples/screenshots
- takes less than 10 minutes to generate ~200 screenshots
- modified /examples/index.html to view "thumbnails", but breaks examples' hash navigations

This affects the examples viewer (and possibly the need to consider storing all the screenshots), but this could probably be tweaked.