Beta release of v3.0.0-beta.12

• Breaking change: Rename the "type" property for signal "bind" specifications to "input".
• Export expressionFunction method for runtime extensibility of expression language functions.
• Add geoArea, geoBounds and geoCentroid expression functions.
• Add rawDomain scale property to dynamically override standard domain definition.
• Add "push": "outer" to push updates from nested mark scopes to signals defined in an outer mark scope.
• Add Vega-Lite selection resolution expression functions.