Update @import behavior per CSSWG resolution

[dandclark]

Last year the CSSWG resolved on the following changes for constructable stylesheets' treatment of @import:

1. insertRule() inserting an @import rule will throw a SyntaxError instead of a NotAllowedError.
2. replaceSync() and replace() both ignore @import rules, and do not throw an exception when encountering one.

This PR updates the spec to reflect these changes.

In this change, the 'ignoring' of @imports for replace()/replaceSync() is achieved by removing the rules from the list produced by the parser. This is a little odd; it could be more natural to have the parser skip them when parsing a constructed stylesheet. But since this document is not yet merged into any other CSS spec it's difficult to plumb that behavior through to the parser. So, the rules are instead removed after-the-fact, which should produce the same result.

Closes #123.

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[dandclark]

This was handled in w3c/csswg-drafts#6411.