Modernized code, PHP 8.1+ & New "duration" capability

Hi!

This release modernizes the codebase, increases the minimum PHP version to 8.1, adds a new "duration" capability and also breaks backwards compatibility (BC) in a few minor ways (hence the new major version!).

PR's

• Modernize Code base by @kbond in #178
• feat: add "duration" formatter by @kbond in #177
• Doc update for 2.0 by @kbond in #180
• minor: use long-form duration message for english by @kbond in #182

CHANGELOG

• Modernize/simplify the code base
• Require PHP 8.1+
• Remove symfony/templating integration
• Make DateTimeFormatter final
• Remove DateTimeFormatter::getDiffMessage()
• Remove DateTimeFormatter::getDateTimeObject()
• Remove DateTimeFormatter::getEmptyDiffMessage()
• Add time_diff Twig filter as an alias for ago
• Add DateTimeFormatter::formatDuration() and duration Twig filter

Full Diff: v1.20.0...v2.0.0

Have fun!