Update selectize.js

[cpsievert]

Update: let's wait on this PR until an official selectize.js release happens (selectize/selectize.js#1613)

• Updates from 0.12.4 to 0.13.3 (https://github.com/selectize/selectize.js/blob/master/CHANGELOG.md)
• Uses the new, native, support for Bootstrap 4 (rather than relying on unofficial ports)
• Builds on the new Bootstrap 4 support support to also support Bootstrap 5 (support was originally added to Shiny in Bootstrap 5 support #3410)

TODO

• The ports were making use of @include border-radius(), but the 'native' version isn't: either make a PR or make a patch.

[cpsievert]

#3875 supersedes this.