Add "meta" field property for tagging support

[gabipetrovay]

We want to add a meta field that will

taggedField: {
    type: "string",
    required: true,
    meta: true,
    labe: "Uau! A field with tagging support"
}

Such a meta option, must enable the following logic in crud and modm:

• an item that has at least one meta field, will have a _mt core schema property (on the same level with _li and _tp) which is a set (i.e. no duplicates) of lower-cased, trimmed strings. For example: _mt: ["value_from_field_1", "value_from_field_2"]
• on every create and update, AND if a meta field is involved, we must rebuild the _mt to avoid gathering trash in it and to gather the new values.