v4.0.0

Thank you @nlemoine for your hard work and refactoring on this new major version (#24):

• Use a PSR16 cache provider
• Update coding standards to PSR12
• Update dependencies
• Remove deprecated methods
• Remove unused/deprecated classes
• Add types
• PHP 8 support
• BC break: MixedFeed does not extend AbstractFeedProvider anymore
• Add raw object in CanonicalItem:

  Canonical items are nice but sometimes, you might need the original response which contains some details that the standard FeeItem doesn't provide