fix(b-table, b-table-lite): handle edge case with row events when table is removed from dom. instantiate row event handlers only when listeners are registered (fixes #4384)

[tmorehouse]

Describe the PR

Fix issue with row-hovered/unhovered events where table is removed from DOM before the event processing gets a chance to finish.

PR also only emits row events if listeners are registered, reducing processing time for events that don't have listeners registered.

Fixes #4384

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.
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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
• 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 #4388 into dev will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4388      +/-   ##
==========================================
+ Coverage   99.91%   99.91%   +<.01%     
==========================================
  Files         239      239              
  Lines        4617     4624       +7     
  Branches     1298     1300       +2     
==========================================
+ Hits         4613     4620       +7     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
src/components/table/helpers/mixin-tbody-row.js	100% <100%> (ø)	⬆️
src/components/table/helpers/mixin-tbody.js	100% <100%> (ø)	⬆️

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[tmorehouse]

It appears that event listeners that are programatically added via this.$on do not appear in this.$listeners, so will need to adjust how the event suppression is handled.

Created an upstream issue at vuejs/vue#10825