Set mimerender index in OutputArea.display_order

[gnestor]

Usage:

OutputArea.register_mime_type(MIME_TYPE, append_json, {
    safe: true,
    index: 1
});

Reference implementation: jupyterlab/mimerender-cookiecutter#9

[gnestor]

@Carreau @takluyver This is a non-breaking change that I'd like to squeeze into 4.3 before releasing. It makes nbextension registration more consistent with labextension.

[takluyver]

Looks OK to me. @Carreau, you added this method, so I'll let you OK the change.

[takluyver]

Oh, nvm. @gnestor made the PR to add it, @Carreau merged. I think this is fine, then. Go ahead and backport it.

[gnestor]

Backported 👍