feat(b-table): better sort labeling for screen readers (closes #4487)

[tmorehouse]

Describe the PR

Switch click to sort .... label from aria-label to an .sr-only element in the field header

If aria-label is present, most screen readers will not read the content of the th cell

This moves the aria-label content into an sr-only hidden span within the header cell.

Closes #4487

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)

One notable change is that if users use labelHtml in a field definition, the html will now be placed into a div element instead of directly in the TH element, so allow for concatenation of the sr-only sort label

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 #4488 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev    #4488   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files         247      247           
  Lines        4855     4855           
  Branches     1363     1366    +3     
=======================================
  Hits         4851     4851           
  Misses          3        3           
  Partials        1        1

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

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