v6.0.0

• Adds Bootstrap 5 support. Note: jQuery is still required
• Removes various IE polyfills
• Replaces var with let
• Normalizes locale strings to IANA standard format (see CHANGLOG).
  • bg_BG -> bg-BG
  • pt-br -> pt-BR
  • zh_CN -> zh-CN
  • zh_TW -> zh-CW