Describe the PR

• Allows directive argument to be a target ID (previously only modifiers and value were checked)
• Allows directive value to be an array of target IDs (handy for toggling multiple collapses)
• Allows directive value to be a space delimited string of target IDs (handy for toggling multiple collapses)
• Adds the class not-collapsed to the trigger element when the target(s) are expanded
• Improves codebase size by removing duplicated code an using utilities where possible

Closes #4834

To Do:

• simplify code by moving applicable utils/target code to v-b-toggle directive code, and remove utils/target since v-b-toggle is the only directive that uses it
• support target as directive argument
• support directive value as a string or array of target IDs
• support passing value as a string of space delimited target IDs
• ensure that duplicate target IDs are filtered out to prevent multiple toggle events
• have v-b-toggle check if evt.defaultPrevented before calling handler(s)
• add class not-collapsed to trigger element when target is not collapsed
• ensures that elements that are not buttons or links have the appropriate tabindex and keydown handlers for accessibility
• directive meta.json update
• switch <b-navbar-toggle> to use v-b-toggle directive
• optimize toggle directive code
• new unit tests (multiple target testing)
• docs update / examples update:
  • v-b-toggle
  • collapse
  • sidebar
  • b-navbar-toggle (to note that it can be used to control sidebars as well, and note that it uses v-b-toggle directive internally)

Future (separate PRs, after this PR is merged):

• add data-class-* sniffing on the trigger element to allow for custom classes for collapsed/not-collapsed' states (Issue Add custom collapse/not-collapsed classes to collapse button #4476)

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
• 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, 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)