Add Code Mirror To Included Scripts

[Shelob9]

A lot of plugins use Code Mirror for code fields. Unfortunately, since it's not included in core, multiple plugins may load conflicting versions, causing issues. At Pods, we have had issues with a Code Mirror conflict with the Debug Bar Console plugin and Soliloquy slider.

If code mirror was included, it could also be used to improve the theme and plugin editors (related discussion in #23601), but that would probably need to be a separate ticket.

[nacin]

We only include scripts that core has a use for, and uses. Otherwise, we need to maintain it. That often means considering backwards compatibility, release philosophies, release timing, the coordination of security vulnerabilities, potential future abandonment or displacement, and the like.