feat(b-navbar-toggle): make default slot scoped

[tmorehouse]

Describe the PR

Makes the default slot of <b-navbar-toggle> scoped, with the scope containing the collapse's expanded state.

Also documents that the component has a default slot

Plus minor code refactor for reduced bundle size

PR checklist

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

• Bugfix
• Feature
• Enhancement
• ARIA accessibility
• Documentation update
• Other (please describe) minor code refactor for reduced bundle size

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, chore(docs): 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
• CodeCov for patch has met target
• 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 #4995 into dev will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4995      +/-   ##
==========================================
- Coverage   99.93%   99.93%   -0.01%     
==========================================
  Files         273      273              
  Lines        6408     6403       -5     
  Branches     1617     1617              
==========================================
- Hits         6404     6399       -5     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
src/components/collapse/collapse.js	100% <ø> (ø)	⬆️
src/directives/toggle/toggle.js	100% <100%> (ø)	⬆️
src/components/navbar/navbar-toggle.js	100% <100%> (ø)	⬆️

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