Run phpunit or pest depending on the project

Remove the `p` and `pf` aliases that previously only worked for PHPUnit,
and re-add them as functions that run either PHPUnit or Pest PHP based
on the project dependencies.

The checking is done within the `phpunit-or-pest` script and then used
within the new `p` and `pf` functions.

bin/phpunit-or-pest

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+if [[ -f "vendor/bin/pest" ]]; then
+  echo "pest"
+  exit 0
+fi
+
+echo "phpunit"