update GitHub workflow to use the latest checkout

adds github actions to run tests and upgrades to php>=7.1 and phpunit…

…^=9.5

Improved the reliability of ArrDots::has

Added Arr::pluck and ArrDots::pluck switch combines column and collapse

Fixed commit error in ArrTest

Added Arr::collapse, Arr::column, and ArrDots::column

Added Collection::implode($glue, $callback)

Added Arr::unique and Collection::unique

[fix] ArrDots::search no longer returns results if the pattern is not…

… found, returns empty array

Added Collection::slice, improved performance of Collection::nth, add…

…ed Collection::find, and removed Collection::toJson