Update Codemirror files

[jeffwidman]

We're currently on a very old version:

flask-debugtoolbar/src/flask_debugtoolbar/static/codemirror/codemirror.js

Line 1 in efe447f

// CodeMirror version 2.23

Latest version is the 6.x series: https://codemirror.net/

Some upgrade notes:

• https://codemirror.net/5/doc/upgrade_v3.html
• https://codemirror.net/5/doc/upgrade_v4.html
• https://codemirror.net/5/doc/releases.html#v5

However, a quick look makes me think upgrading should be relatively straightforward.

Alternatively, we could switch to a different editor, but I suspect that'd be significantly more work.