Fix some JS todos and warnings

[mdo]

Resolves outstanding v6 todo comments and some eslint warnings.

Todos

File	Change
js/src/carousel.js	Changed interval type from '(number|boolean)' to 'number'
js/src/carousel.js	Replaced document.hidden with document.visibilityState === 'visible'
js/src/carousel.js	Removed TODO comment about empty divs test
js/src/dom/event-handler.js	Removed outdated TODO about tooltip passing false
js/src/dom/selector-engine.js	Removed unused next() method
js/src/dropdown.js	Simplified menu lookup to just use findOne() within parent
js/src/scrollspy.js	Removed deprecated offset option and related code
js/src/tab.js	Simplified SELECTOR_DATA_TOGGLE to only '[data-bs-toggle="tab"]'
js/src/tab.js	Changed to throw TypeError when parent is invalid instead of silent return
js/src/tooltip.js	Removed null check and TODO comments for tip element
js/tests/unit/dropdown.spec.js	Removed "invalid markup" test

ESLint

File	Issue	Fix
js/src/dropdown.js	Complexity warning (22 > 20)	Extracted escape key handling into new _handleEscapeKey() method
js/tests/unit/dropdown.spec.js	Max nested callbacks (5 > 4)	Refactored test to use async/await pattern