Add arrow

[camskene]

'Velcro is designed to "create an experience out of the box that is useful" - to that end, adding an arrow to a tooltip or popover should be as simple as possible and easy for consumers to style.

See https://floating-ui.com/docs/arrow

[NullVoxPopuli]

I have an implementation of this over here:
https://github.com/universal-ember/ember-primitives/pull/38/files#diff-923252e42afb249fe91098b5fe111e16c1cb59a766ef2d478e695c6e4d353bd5R41

Code: https://github.com/universal-ember/ember-primitives/pull/38/files#diff-ed5e9331439bf50c215f4e7829854af5098476ffa6d12c4848117123efdc4fc0R180

Had to do another modifier, because it's an opt-in thing.
There is also a problem of double-render, because the floating-ui is setup when only the hook and loop have elements, but there isn't a way I could think of to somehow know to wait for the third element before booting up.

Something else I noticed is that the middleware data is frequently dirty -- even when the values don't change. I think this would be an issue for floating-ui to fix, because it doesn't make sense for the autoUpdate function to call again if the data isn't changing, ya know?

[NullVoxPopuli]

Demo here: https://ember-primitives.pages.dev/3-components/popover