Extract codemirror SQL editor out into a plugin

[simonw]

Right now codemirror (used for the SQL editor on https://latest.datasette.io/fixtures?sql=select+*+from+%5B123_starts_with_digits%5D ) is the only JavaScript in Datasette.

It's also the only vendored dependency.

I'd like to move it out to a plugin. But... ideally I would like that plugin to be part of the default "pip install datasette" experience.

I don't know what the best pattern for optional dependencies is. I don't want to have to tell people to run pip install datasette[full]

[simonw]

I've changed my mind - especially since Datasette now ships with default JavaScript for the column menus in #981. I'm not going to extract CodeMirror into a plugin, I'll leave it where it is.