[WIP] Add Bootstrap 5 support

[IanDelMar]

This PR adds basic Bootstrap 5 support to the walker.

Fixes #528

Changes proposed in this Pull Request:

• Adds the possibility to request Bootstrap 5 markup via the arguments passed to the wp_nav_menu() function.
• Adds the bs infix to data attributes if Bootstrap 5 markup is requested.
• Adds the .active class to the a element instead of the li element if Bootstrap 5 markup is requested.

Proposed changelog entry for your changes:

Add basic Bootstrap 5 support

Notes:

• Unit tests are missing - but I suggest to add them later in another PR
• This PR builds upon Fix placement of the .active class for dropdown items & remove dropdown when item has no visible children #530
• Todo:
  • add support for .visually-hidden

Related issues and PRs:

#499, #513, #500

[ethanclevenger91]

@IanDelMar What's the status of this PR? You seem fairly interested in maintenance on this project. Would it be worth a maintained fork for BS5 and continued development? Unless @pattonwebz has plans to return to this.

[leqnam]

This PR should be merged.