After dropping IE11: A tracking issue

[RunDevelopment]

This issue is for keeping track of enhancements/changes that should be made immediately after we drop IE11.

This issue is not for discussing whether, when, or how we should/will drop IE11 (see #1578 for that).

Tasks

Core

• Use Element#closest to implement Prism.util.isActive. (see comment)
• Remove Prism.util.currentscript and use document.currentScript instead. (see Core: IE11 workaround for currentScript #2104)

Plugins

• Promise-based implementation for Autoloader (Rewrote Autoloader using Promises #1978)