feat: ember 3.16+ support

[matthewhartstonge]

Howdy!

We've got a rather large app that makes use of this addon and am trying to punch up to Ember@5. (we have a lot of HTML+CSS to untangle sitting in sticky elements)

This is the start of where I'm at to get us compiling due to relying on ember-in-viewport. Thought I would punch up what I've done so far.

Viewport doesn't seem to be firing 'well' so I've had to do a dirty and rely on debouncing on scroll, but that still doesn't seem to fix the boxes sticking to the bottom in the dummy app.

I'm a backend dev attempting to upgrade our frontend app, so apologies if I'm committing heinous sins.

1..183
# tests 183
# pass  171
# skip  0
# todo  0
# fail  12