Use expressions while calculating values

[kbarbounakis]

Is your feature request related to a problem? Please describe.
@themost/query provides methods of calculating literal values by defining expressions like

"$expr": {
    "$round": [
        14567.5671,
        2
    ]
}

DefaultValueListener and CalculatedValueListener use FunctionContext to calculate values by using native javascript closures but this operation should be improved and try to use @themost/query approach of calculating literal values.

Describe the solution you'd like
Use @themost/query for defining default or calculated values e.g.

{
            "@id": "https://themost.io/schemas/dateModified",
            "name": "dateModified",
            "title": "dateModified",
            "description": "The date on which this item was most recently modified.",
            "type": "DateTime",
            "readonly": true,
            "calculation": {
                  "$expr": {
                        "$now": []
                   }
            }
        }