Selectors: benchmarks `phf` vs. binary search for case insensitive attribute lookup

[nicoburns]

https://bugzilla.mozilla.org/show_bug.cgi?id=2042048 introduces a new slower-but-lighterweight code path for detecting case insensitive attributes and gates the existing phf-based implementation behind an on-by-default feature flag.

We should benchmark the new binary_search-based method against the phf method to see if we can remove the PHF-based lookup entirely.