We should probably disallow inserting @importrules into constructed stylesheets or any of their imports

[bzbarsky]

If someone does https://drafts.csswg.org/cssom/#dom-cssstylesheet-insertrule on a constructed stylesheet, or any of its imports (or their imports, etc) and the parsed rule is an @import rule, we should probably throw. Especially if we want to be able to share the sheets across documents.

[rakina]

Fixed by #61.