Use orderby, not order. Allow highlight to automatically set orderby.

[jheer]

This PR fixes a bug in which the highlight interactor may highlight the wrong items if the data ordering of the visualized data is inconsistent with highlight query results. A workaround was to add sort criteria to ensure consistent ordering. This PR updates the highlight interactor to automatically add appropriate orderby criteria to a highlighted mark when needed.

Breaking change: This PR also addresses a conflict where the mark-level order option (which specified query orderby criteria) can be confused with Observable Plot's order option, which parameterizes internal stacking transforms. This update now uses a non-conflicting orderby option, leaving order as a Plot-specific option. Any existing specifications which use the order option at the mark-level will likely need to update to use orderby instead.

Fix #248.