This fixes a few issues:
- strict mode was producing false negatives if multiple query paths
lead to the same element being picked;
- in some cases the number of intermediate items in the list was
exponential and crashed quickly.
What changed:
- `visible` engine is a real engine now;
- `capture` selectors are transformed to `has=` selectors for
easier implementation;
- chained querying switched from a list to a set to avoid
exponential size.
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