fix(b-carousel): transition when controls are inside slides (closes #5361)

[Hiws]

Describe the PR

This is an attempt to fix the broken transition that appears when placing navigation controls inside a slide, that either calls the prev or next methods or changes the v-model on the <b-carousel>.

I've ran through the example page, and the carousels appear to be working correctly.
However it might require more testing to ensure that this change doesn't break anything.

Closes #5361

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix (fixes a boo-boo in the code) - fix(...), requires a patch version update
• Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
• Enhancement (augments an existing feature) - feat(...), requires a minor version update
• ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• Other (please describe)

Does this PR introduce a breaking change? (check one)

• No
• Yes (please describe since breaking changes require a minor version update)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• It should address only one issue or feature. If adding multiple features or fixing a bug and adding a new feature, break them into separate PRs if at all possible.
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, chore(docs): fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages, and determines the next version type (patch or minor).

If new features/enhancement/fixes are added or changed:

• Includes documentation updates
• Includes component package.json meta section updates (prop, slot and event changes/updates)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (required for new features and enhancements)
• Existing test suites are passing
• CodeCov for patch has met target (all changes/updates have been tested)
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (Does it affect screen reader users or keyboard only users? Clickable items should be in the tab index, etc.)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

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[codecov]

Codecov Report

Merging #5462 into dev will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               dev     #5462   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          277       277           
  Lines         9015      9015           
  Branches      2496      2496           
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  Hits          9015      9015           

Flag	Coverage Δ
#unittests	100.00% <ø> (ø)

Impacted Files	Coverage Δ
src/components/carousel/carousel.js	100.00% <ø> (ø)

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[Hiws]

For reference, this is the code I used to test with on the playground.

<b-carousel ref="myCarousel" indicators :interval="0" @sliding-end="slidingEnd">
  <b-carousel-slide caption="First slide" img-src="https://picsum.photos/1024/480/?image=10">
    <b-button @click="next"> next </b-button>
  </b-carousel-slide>
  <b-carousel-slide caption="Second Slide" img-src="https://picsum.photos/1024/480/?image=12">
    <b-button @click="prev"> prev </b-button>
    <b-button @click="next"> next </b-button>
  </b-carousel-slide>
  <b-carousel-slide caption="Third Slide" img-src="https://picsum.photos/1024/480/?image=22">
    <b-button @click="prev"> prev </b-button>
  </b-carousel-slide>
</b-carousel>

<script>
{
  methods: {
    prev() {
      this.$refs.myCarousel.prev();
    },
    next() {
      this.$refs.myCarousel.next();
    },
    slidingEnd() {
      console.log('sliding end')
    }
  }
}
</script>

[jacobmllr95]

@tmorehouse I've created a CodeSanbox with the changes of this PR and can confirm that is resolves the problem: https://codesandbox.io/s/sharp-chebyshev-j2u0y