Describe the PR

Emit new event activate-tab before changing tabs.

New event is emitted with three arguments:

• newTabIndex tab index that will be activated
• prevTabIndex tab index of the current active tab, or -1 if no active tab
• bvEvt BvEvent object instance

Calling bvEvt.preventDefault() will prevent the tab from activating.

Ideally users should be setting the disabled prop on any <b-tab> they don't want users to activate (better for accessibility). If users do use this event to prevent a tab from activating, they should provide feedback to the user as to why the tab cannot be activated (i.e. an alert, etc).

Closes #4273

To Do:

• meta data update
• Get event working with v-model/value update
• documentation update
• unit tests

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)