fix(b-table): default role to grid when selectable and table otherwise

[andrei-gheorghiu]

Describe the PR

Resulting <table> of <b-v-table> should have a default role of table.
If selectable is true, the role applied should be grid.

In both cases, user-provided role has priority.

Related: #6305

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)

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[codecov]

Codecov Report

Merging #6383 (344029a) into dev (9e25a3b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev     #6383   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          299       299           
  Lines        10141     10142    +1     
  Branches      2524      2522    -2     
=========================================
+ Hits         10141     10142    +1     

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/table/helpers/mixin-selectable.js	100.00% <100.00%> (ø)
...c/components/table/helpers/mixin-table-renderer.js	100.00% <100.00%> (ø)

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[jacobmllr95]

Thanks @andrei-gheorghiu!