Dynamically set/remove `aria-modal="true"` when a modal is shown/hidden

Having variables initialised from start `_isTransitioning` is better.
Would be better to add an eslint rule to check for undeclared variables use.
Reordered enter checks for `show` and `hide` by priority.

I'm using Tab.js with remove function and get an error "TypeError: container is undefined [more info]", with this check the error is fixed.

in particular, no need to use originEvent, and preventDefault() only needed for touch events

these may also be the cause of weird behavior in Chrome/Surface, where scrolling vertically triggers slide advance

adds the ability to assign data-interval to an individual carousel-item

see: https://github.com/twbs/bootstrap/issues/26847
in v4, destroy was replaced by dispose