feat(b-dropdown): add splitClass property to dropdown component

[enriquecastl]

Describe the PR

Adds a splitClass property to the dropdown component to allow setting a custom class selector to the dropdown split button when split mode is enabled.

Why is this necessary?

We need to customize the dropdown’s split and toggle buttons based on our design system specifications. We apply these customizations by adding a custom class selector using the toggleClass property for the toggle button. We also want to apply a class selector to the split button, but the dropdown component don’t expose a property to achieve that.

We are using a workaround of applying these classes when the component is updated, but we’d like to replace it with a sound approach.

PR checklist

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

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe)

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

• No
• Yes (please describe)

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e. [...] (fixes #xxx[,#xxx]), where "xxx" is the issue number)
• 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.
• The title should follow the Conventional Commits naming convention (i.e. fix(alert): not alerting during SSR render, docs(badge): update pill examples, fix typos, chore: fix typo in README, etc). This is very important, as the CHANGELOG is generated from these messages.

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

• Includes documentation updates (including updating the component's package.json for slot and event changes)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• 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)

[codecov]

Codecov Report

Merging #4394 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #4394   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files         240      240           
  Lines        4622     4622           
  Branches     1303     1303           
=======================================
  Hits         4618     4618           
  Misses          3        3           
  Partials        1        1

Impacted Files	Coverage Δ
src/components/dropdown/dropdown.js	100% <ø> (ø)	⬆️

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[jackmu95]

Thanks @enriquecastl!