When testing with local server, show editor with more realistic width

[schalkneethling]

Duplicate of mdn/interactive-examples#811 - work on this will happen here.

Original report from @chrisdavidmills

I just thought — when writing the interactive examples we always try to make both the example and the code lines narrow enough so they fit comfortably inside their respective panes.

But when you text it locally, the editor is shown at the full screen width, which is a lot wider than it'll actually be shown in the content column on MDN.

It would be great to have an option to set it to the typical MDN content column size instead. I could resize my browser, but I'm lazy ;-)

Latest comment by @stephanmax - mdn/interactive-examples#811 (comment)

Just realized that this is more complicated than I thought.

My initial plan was to bundle in some CSS with the editor styles that makes sure the code demos are embedded in a more realistic surrounding. But if I understand mdn-bob correctly, this bundled CSS would also end up in the production demos.

What do you think @schalkneethling, can we add an env variable to the npm scripts to only add the mdn mock CSS when building locally/testing code demos?