fix(b-form-tags): ensure same height with or without tags (closes #5750)

[jacobmllr95]

Describe the PR

This PR ensures that <b-form-tags> has the same height with or without tags.

The issue was introduced with #5544 which removed the top margin from the field <li> element which holds the input and the add button. Since this element is always rendered, other than the tags, adding the same top margin as the tags ensures it always balances the negative margin of the tags list element.

While at it, this PR unifies the safeId() and element keys.
It also fixes a small a11y where the aria-describedby of <b-form-tags> pointed to the wrong ID which was introduced during the changes made in #4775.

Closes #5750.

PR checklist

What kind of change does this PR introduce? (check at least one)

• Bugfix (fixes a boo-boo in the code) - fix(...), requires a patch version update
• Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
• Enhancement (augments an existing feature) - feat(...), requires a minor version update
• ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• Other (please describe)

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

• No
• Yes (please describe since breaking changes require a minor version update)

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, and determines the next version type (patch or minor).

If new features/enhancement/fixes are added or changed:

• Includes documentation updates
• Includes component package.json meta section updates (prop, slot and event changes/updates)
• Includes any needed TypeScript declaration file updates
• New/updated tests are included and passing (required for new features and enhancements)
• Existing test suites are passing
• CodeCov for patch has met target (all changes/updates have been tested)
• 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)

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/bootstrap-vue/bootstrap-vue/2tf4se7fn
✅ Preview: https://bootstrap-vue-git-fix-b-form-tags-height-when-empty.bootstrap-vue.vercel.app

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 8ad6233:

Sandbox	Source
BootstrapVue Starter Project	Configuration

[codecov]

Codecov Report

Merging #5752 into dev will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev     #5752   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          284       284           
  Lines         9569      9569           
  Branches      2525      2525           
=========================================
  Hits          9569      9569           

Flag	Coverage Δ
#unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/components/form-tags/form-tags.js	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e67d341...8ad6233. Read the comment docs.