feat(b-button-close): add content prop (closes #4527)

[jacobmllr95]

Describe the PR

This PR adds a content prop to the <b-button-close> component.
It also adds the prop to the BButtonClose settings to make it globally configurable.

A headerCloseContent property is also added to the <b-modal> component and the BModal settings to be able to change the close button content by modal or for all modals globally.

Closes #4527.

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, 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 #4574 into dev will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #4574      +/-   ##
==========================================
- Coverage   99.92%   99.92%   -0.01%     
==========================================
  Files         251      251              
  Lines        5225     5220       -5     
  Branches     1384     1384              
==========================================
- Hits         5221     5216       -5     
  Misses          3        3              
  Partials        1        1

Impacted Files	Coverage Δ
src/utils/dom.js	100% <100%> (ø)	⬆️
src/utils/array.js	100% <100%> (ø)	⬆️
src/utils/object.js	100% <100%> (ø)	⬆️

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[tmorehouse]

One thing to note is that we used to have SSR mismatch errors when using × from a prop for some reason (there was a related Vue issue for that, which might be fixed by now).

When using HTML entities that maps to UTF8 characters, the browser would convert the inner HTML/text to the UTF8 character, and the rehydration process would complain that × does not equal ×, even though they are.

[jacobmllr95]

@tmorehouse I currently don't see any SSR mismatch errors.

[tmorehouse]

@jackmu95 OK cool... Vue must have fixed the issue with the rehydration process.

[tmorehouse]

I might add in an option for passing vNode Array via the message box methods for the button content (via a separate PR)

[tmorehouse]

We target this for the v2.3.0 release (after any v2.2.1 release)