Add dedicated undo/redo keyboard utilities

[nikku]

The way folks in the bpmn-io sphere may now test keyboard events against common behaviors is now greatly simplified:

import {
  isUndo,
  isRedo,
  isPaste
} from 'diagram-js/lib/features/keyboard/KeyboardUtil';

document.addEventListener('keydown', function(event) {
  if (isUndo(event)) {
    // event is regarded as an UNDO event by bpmn-io tools.
  }
});

As a breaking change we stop exporting some basic utilities in KeyboardBindings (exporting from actual feature modules is an anti-pattern anyway) with more to be removed in future major upgrades. The remaining ones are continued to be used in downstream libraries (mainly form-js).

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Non-breaking changes backported to:

• 7.x - feat(keyboard): make function detection available via explicit utilities #663
• 8.x - feat(keyboard): make function detection available via explicit utilities #664