Add PHP 8-only support (v2)

[patinthehat]

This PR adds a new major version, v2.0.0.

Specifically, it:

• Removes support for all PHP 7.x versions.
• Requires PHP 8.0+.
• All syntax converted to PHP 8 where possible.
• Removes unnecessary PHP docblocks per Spatie's guidelines.
• Moves exception classes to an "Exceptions" directory to match the structure of other packages.
• Adds config & github workflow for php-cs-fixer.
• Updates the location of CONTRIBUTING.md.
• Updates the readme with minor adjustments from spatie/package-skeleton-php.
• Updates the changelog - release date needs to be updated, currently is "unreleased".

[patinthehat]

ping @freekmurze

[freekmurze]

Thanks!