Encoding's `datum` for constant values in data domain that still pass through scales

[kanitw]

value in fieldDef set the range value directly.

If users wanna provide domain value that needs to be scaled, we need to add another property.
From a conversation long time ago, we decided that this property is datum.

To clarify, a value property in VL produces a Vega ValueRef {value: value}.
Meanwhile, a datum property in VL should produces a Vega Value Ref {scale:..., value: datum} and use scale from relevant layers.

cc: @swojit

[kanitw]

• Change Dropping condition in unit.ts