Update Codemirror files

We're currently on a very old version: https://github.com/pallets-eco/flask-debugtoolbar/blob/efe447fb5fa53258c2e289c579179542bfa0036d/src/flask_debugtoolbar/static/codemirror/codemirror.js#L1 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.