add modal my gawd

Closes #16073 by merging it

To ensure that we don't accidentally use any of the aliases.
This should prevent any future regressions from #12761.
Also updates the test suite since it now can't use these aliases either.

Appears to be unnecessary and is broken by #16011,
which is otherwise quite sane.

Thanks to @peterblazejewicz & @Nikita240
Adds unit tests based on #14417
X-Ref: #14282
Closes #15807

[Fixes #15953] Implement `assert.expect` in each unit test

Fixes #15136.
Closes #15345.
Closes #15314.
Refs #14724, #14927.

When the first two elements in a scrollspy content block have a document
offset of zero (i.e. they're hard against the top of the page),
Scrollspy would switch between them on every scroll event.

This could happen, for example, in a system of nested sections:

```
<section id="animals">
  <section id="dogs">
	Content
  </section>
</section>
```

This ocurred because Scrollspy's check to see if it's at the end of the
array of sections uses `!arr[index]`. This misses the case where
`arr[index]` does exist and is zero.

This commit explicitly checks the array bounds.

Fixes #15741
Closes #15751 by merging it.

Refs http://qunitjs.com/upgrade-guide-2.x/#replace-qunit-jsdump-with-qunit-dump

Refs #15893

Fixes #15147.
Closes #15520 by merging it.

Redo of #15876

Add tests for `aria-hidden` of modal

Use strictEqual of 1 here instead of 2 asserts

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Closes #15484

This reverts commit 1b323762.
This reverts PR #14189 because it caused major regressions.

Fixes #15168.

We'll try to revisit #14167's feature request in Bootstrap v4.

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Special thanks to @programcsharp

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