feat(tooltip, popover): add support for contextual variants and custom class (closes #1983, #2075)

[tmorehouse]

Describe the PR

• Add support for tooltip and popover variants
• Add support for custom class for tooltips and popovers
• Improve directive version performance by pre-compiling modifier test regular expressions

New props on components:

• variant - supports Bootstrap theme colors (i.e. 'danger', 'primary', 'info', etc)
• custom-class - a string classname added to the outer wrapper of the tooltip or popover.

Directive versions have new modifier v-<variant>, and support passing optional config options variant and className (passing config object as the expression)

Default values (current value is null) can be specified via the BootstrapVue global config.

SCSS Notes:

• Tooltip variants use the base Bootstrap theme colors for background, and yiq-color for text.
• Popovers also use the bootstrap theme colors and borrows from alert styling for the background, border and text color.
• If users define new theme colors and compile Bootstrap + BootstrapVue SCSS, the new themes will kick in.
• Users can optionally disable generation of tooltip and/or popover variants by setting variables in SCSS

Closes: #1983, #2075

To Do:

• new unit tests
• tooltip SCSS
• popover SCSS
• update b-tooltip/b-popover for variant / custom-class
• update v-b-tooltip/v-b-popover for variant / custom-class
• documentation update

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, fix typos, chore: 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)
• 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 #3644 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #3644      +/-   ##
==========================================
+ Coverage   99.26%   99.26%   +<.01%     
==========================================
  Files         224      224              
  Lines        4326     4346      +20     
  Branches     1227     1229       +2     
==========================================
+ Hits         4294     4314      +20     
  Misses         26       26              
  Partials        6        6

Impacted Files	Coverage Δ
src/mixins/toolpop.js	100% <ø> (ø)	⬆️
src/components/tooltip/tooltip.js	100% <100%> (ø)	⬆️
src/utils/tooltip.class.js	92.44% <100%> (+0.08%)	⬆️
src/directives/popover/popover.js	100% <100%> (ø)	⬆️
src/directives/tooltip/tooltip.js	100% <100%> (ø)	⬆️
src/components/popover/popover.js	100% <100%> (ø)	⬆️

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