feat(b-calendar): add nav-button-variant prop (closes #5702)

[Hiws]

Describe the PR

This PR adds a nav-button-variant prop to <b-calendar> and <b-form-datepicker>, to control the variant.
It defaults to secondary, and is configurable via <b-calendar> and <b-form-datepicker> settings.

The PR also adds support for the selected-variant and today-variant props to be configurable via settings.

Closes #5702.

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.)

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[codecov]

Codecov Report

Merging #5705 into dev will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev     #5705   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          284       284           
  Lines         9547      9548    +1     
  Branches      2520      2520           
=========================================
+ Hits          9547      9548    +1     

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

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/components/calendar/calendar.js	100.00% <ø> (ø)
src/components/form-datepicker/form-datepicker.js	100.00% <100.00%> (ø)

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