feat(b-calendar, b-form-datepicker): new components b-calendar and b-form-datepicker (closes #3676, #1428)

[tmorehouse]

Describe the PR

New components:

• <b-calendar> calendar widget component
• <b-form-datepicker> date picker form input component (based on <b-calendar> and <b-dropdown>)

Calendar supports internationalization via Intl.DateTimeFormat, but requires aria labels to be manually internationalized.

Components are WAI-ARIA compliant. Both components are minimalistic by design for ARIA simplicity (i.e. no date range selection).

<b-calendar> and <b-form-datepicker> require minor custom CSS.

BCalendar:

https://deploy-preview-4712--bootstrap-vue.netlify.com/docs/components/calendar/

BFormDatepicker (collapsed state):

https://deploy-preview-4712--bootstrap-vue.netlify.com/docs/components/form-datepicker/

Closes #3676
Closes #1428

Screen reader tests manually performed with ChromeVox and NVDA

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, 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 #4712 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4712      +/-   ##
==========================================
+ Coverage   99.92%   99.92%   +<.01%     
==========================================
  Files         256      261       +5     
  Lines        5292     5679     +387     
  Branches     1405     1480      +75     
==========================================
+ Hits         5288     5675     +387     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
src/components/index.js	100% <ø> (ø)	⬆️
src/index.js	100% <ø> (ø)	⬆️
src/components/modal/modal.js	99.59% <ø> (ø)	⬆️
src/components/table/helpers/filter-event.js	100% <ø> (ø)	⬆️
src/mixins/dropdown.js	100% <100%> (ø)	⬆️
src/components/dropdown/dropdown.js	100% <100%> (ø)	⬆️
src/components/calendar/index.js	100% <100%> (ø)
src/components/form-datepicker/index.js	100% <100%> (ø)
src/components/nav/nav-item-dropdown.js	100% <100%> (ø)	⬆️
src/components/calendar/calendar.js	100% <100%> (ø)
... and 7 more

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[epileftro85]

Finally, these are great news, @tmorehouse thanks for the awesome work

[tmorehouse]

While working on the b-date-datepicker component validation states, I discovered a bug in Bootstrap v4.4 CSS regarding input-group sizing and form-control validation states: twbs/bootstrap#30131