Required new .btn-group-input class

That's likely a deal breaker on this being in v4 as an update to the existing button group. We'd have to deprecate the existing one and provide this as a new component. Breaking changes are a no-go heading into stable v4.0.

I added .btn-group-input as a class similar to .custom-control-input, but It can be realized without .btn-group-input by complicating CSS.

.btn-group-input { ... }

to

.btn-group > input[type="radio"],
.btn-group > input[type="checkbox"] { ... }

<div class="btn-group">
  <input type="checkbox" id="checkbox1">
  <label class="btn btn-primary" for="checkbox1">Single toggle</label>
</div>

i'd like to see this addressed in 4.1 perhaps though. toggle groups are extremely brittle due to their reliance on JS which makes them easily go out of sync with things like keyboard behavior, autofill behavior, etc

x-ref: #25281

I like this, but maybe we better move this to v5? Otherwise we'll have 2 implementations of the same component. @mdo, what do you think?

Superseded by #28463.