Add Bootstrap 5 support

[henning-unicode]

Initial Bootstrap 5 support. Should work for level 1 navigation with dropdown menus with one level. More levels, separators and other special cases need to be tested.

[henning-unicode]

@n-rodriguez I couldn't successfully test this with the example with all features in the Readme. Not sure if this is a problem with the new code, the example or the setup of the application I am testing it with. Feel free to test it out and review if you have time. Otherwise I will try to get the test of the different features running some time soon.

[henning-unicode]

@n-rodriguez would be great if you could review this PR :) Tests are missing, but they also are missing for the Bootstrap 4 renderer... sorry there is still a bug, I try to fix is ASAP

[henning-unicode]

@n-rodriguez it's ready for review now

[n-rodriguez]

Thanks for this contribution! Bootstrap 5 support has just landed on master in commit 13c020a — a full renderer (bootstrap5.rb / rendered_item5.rb) registered as :bootstrap5, with dropdown menus, dividers, headers, split toggles and parity characterization specs, plus README/CHANGELOG updates (version bumped to 1.4.0). Closing this PR as the feature is now implemented. Thanks again for kicking it off!