WIP rewrite for Craft 4

[ostark]

Discussions https://github.com/ostark/upper/discussions/68 & https://github.com/ostark/upper/discussions/65 Issue https://github.com/ostark/upper/issues/67

So far no significant code changes, just structural.

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• [x] Prepare for tests with pest
• [x] Use Handler classes instead of closures
• [x] Use DI Container instead of Service Locator (not everywhere so far)
• [x] Move event listener setup to the main Plugin class (remove EventRegistrar)
• [ ] Refactor drivers (using https://github.com/Sammyjo20/Saloon)

[mattbloomfield]

Hey @ostark any progress here?