Revise syntax for using signal in an encoding constant `value`.

Seeing datum signal in https://github.com/vega/vega-lite/pull/6432 as encoding.color = {datum: {signal: ...}

I think we should revise encoding.color = {signal:...} to be encoding.color = {value: {signal:...}} so it’s consistent and easier to teach, though a bit more verbose.

Basically, once we have signal as an official feature, we can just say value is a constant value in visual domain and datum is constant value in data domain, both of which support signals as their values. (Instead of saying value signal use {signal:...} but datum signal needs nesting.)

[domoritz]

{value: 5, scale: true} makes sense to us but I think is a lot more confusing compared to datum.

[kanitw]

will be fixed in https://github.com/vega/vega-lite/pull/6438