feat(spinner): Pre-Release Bootstrap V4.2 spinners

[tmorehouse]

Description of Pull Request:

Adds pre-release support for Boostrap V4.2 spinners via a new component <b-spinner>

Built as a Vue functional component, so it is very lightweight.

Conditionally includes spinner SCSS styles (will auto-exclude once V4.2 is released)

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:

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

• Yes
• No

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 #2306 into dev will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #2306      +/-   ##
==========================================
+ Coverage   67.15%   67.24%   +0.09%     
==========================================
  Files         162      164       +2     
  Lines        3218     3227       +9     
  Branches      894      896       +2     
==========================================
+ Hits         2161     2170       +9     
  Misses        778      778              
  Partials      279      279

Impacted Files	Coverage Δ
src/components/spinner/index.js	100% <100%> (ø)
src/components/spinner/spinner.js	100% <100%> (ø)

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