feat(core): create configurable base global configuration

[tmorehouse]

Description of Pull Request:

Adds the ability to provide configuration options for BootstrapVue. i.e. ability to specify breakpoint names (if different from default bootstrap v4), or what the default variants (or test strings) for various components are.

When defining custom breakpoint names, the breakpoint specific props of <b-col> and <b-form-group> will be renamed to match the provided new breakpoint names.

I.e. if the new breakpoints are ['aa', 'bb', 'cc', 'dd'] then <b-col> will now have bb, cc, and dd props instead of sm, md, lg and xl. Similar for the label-cols-{breakpoint} and label-align-{breakpoint}props on <b-form-group>.

Also adds in an option to document deprecated properties, but setting a prop definition deprecated property to either true or to a string deprecation message:

props {
  foo: {
    type: String,
    default: null,
    deprecated: true
  },
  bar: {
    type: String,
    default: null,
    deprecated: '"bar" prop is deprecated. Use "foobar" instead'
  },
  foobar: {
    type: String,
    default: null
  }
}

This shows up in the component prop reference section as a row-details row:

ToDo:

• b-form-group get breakpoint names from config
• b-col get breakpoint names from config
• b-alert get default variant from config
• b-badge get default variant from config
• b-button get default variant from config
• b-button-close get default text-variant from config
• b-dropdown get default button variant from config
• b-form-file get default placeholder and browse text from config
• b-img get default blank-color from config
• b-img-lazy get default blank-color from config
• b-modal get button default variants and text from config
• add config option to each individual plugin
• update settings reference section
• Allow config to be passed via Nuxt.js plugin module

PR checklist:

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

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe: code updates

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact:

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 #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR 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.
• PR titles should following the
  Conventional Commits naming convention (i.e.
  "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos",
  "chore: fix typo in docs", 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)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• 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 #2905 into dev will increase coverage by 0.23%.
The diff coverage is 99.5%.

@@            Coverage Diff             @@
##              dev    #2905      +/-   ##
==========================================
+ Coverage   90.79%   91.03%   +0.23%     
==========================================
  Files         202      205       +3     
  Lines        3237     3290      +53     
  Branches      973      996      +23     
==========================================
+ Hits         2939     2995      +56     
+ Misses        215      213       -2     
+ Partials       83       82       -1

Impacted Files	Coverage Δ
src/components/carousel/index.js	100% <ø> (ø)	⬆️
src/components/alert/index.js	100% <ø> (ø)	⬆️
src/components/form-select/form-select.js	100% <ø> (ø)	⬆️
src/components/spinner/index.js	100% <ø> (ø)	⬆️
src/directives/modal/index.js	100% <ø> (ø)	⬆️
src/components/carousel/carousel.js	95.96% <ø> (ø)	⬆️
src/components/pagination/index.js	100% <ø> (ø)	⬆️
src/directives/scrollspy/index.js	100% <ø> (ø)	⬆️
src/components/image/index.js	100% <ø> (ø)	⬆️
src/components/form/index.js	100% <ø> (ø)	⬆️
... and 60 more

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