General reworking to include `spatie/async` behavior without restrictions, issues.

• Updated "how to integrate" section, and documented differences with original author's spatie/async package.
  • Some of these changes was part of previous fork PR spatie/async#56, others fix or address many issues raised in: spatie/async#168, spatie/async#147, spatie/async#162, spatie/async#164, spatie/async#168, spatie/async#160, spatie/async#153, spatie/async#152, spatie/async#157, spatie/async#134, spatie/async#76, spatie/async#107, spatie/async#98, spatie/async#4.
• Removed global keyword usage, using static class Globals to maintain/passing child process GLOBALS like ext-parallel thread extension behaves.