refactor(GlCollapse): remove `BCollapse` component

BREAKING CHANGE:

`GlCollapse` now has its own implementation and does not rely on
`BCollapse` anymore.

Drops support of the `toggle` directive for toggling the `GlCollapse`'s
state. Going forward, the `v-model` should be used instead.
Drops support of `hide`, `show` events.

Also drops supports of:
- `appear` prop
- `v-b-toggle`, which can be used to trigger multiple collapse elements
- turn a group of collapse components into an accordion with `accordion`
 prop
- navbar support with `is-nav` prop
- $root event
- `show` and `hide` events

See bootstrap doc
(https://bootstrap-vue.org/docs/components/collapse#collapse) for
details.