[v3] add type hints to function signatures

[stklcode]

based on #575, #576 and #579 (all merged)

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• raise required PHP version from 7.0 to 7.2
  We have nullable and void return types, so we can annotate almost every method (some union and mixed types missing, those are PHP 8 features)
• add type hints where possible
• remote redundant type checks in method bodies
• use null coalescing operator (??) where applicable

Fixed 2 minor bugs that came up during this refactoring:

• add missing return statement to Linkback::process()
• ensure $implements_interface is initialized in ComponentHelper