feat(b-calendar, b-form-datepicker): add optional decade navigation buttons (addresses #4976)

[tmorehouse]

Describe the PR

Adds new prop show-decade-nav to enable optional +/- decade navigation

Addresses parts of #4976

To Do:

• b-calendar unit tests
• update b-form-datepicker
• b-calendar docs
• b-form-datepicker docs

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 #5112 into dev will not change coverage by %.
The diff coverage is n/a.

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##              dev    #5112   +/-   ##
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  Coverage   99.95%   99.95%           
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  Files         277      277           
  Lines        6522     6522           
  Branches     1612     1612           
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  Hits         6519     6519           
  Misses          3        3           

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[tmorehouse]

@jackmu95 Do you think the decade buttons should be opt-in (as current in this PR, set prop to show), or opt-out (shown by default, set prop to hide)?

Current opt-in demo: https://deploy-preview-5112--bootstrap-vue.netlify.com/docs/components/calendar#internationalization

We could make the decade buttons automatic by default (depending on the range for min/max). basically a tri-state prop... null for auto, true for always show, and false for never show?