Add init type

[vitaly-t]

Consider refactoring value into a strict type:

readonly init: (value: any) => any;

[vitaly-t]

Implemented in 10.5.7.

And type ColumnSet now supports templating for the source in methods init and skip:

interface IMyType {
    val: number;
}

const cs = new pgp.helpers.ColumnSet<IMyType>([
  {
    name: 'my-col',
    init(c) {
        // c.source is now strongly-typed as IMyType
    },
    skip(c) {
        // c.source is now strongly-typed as IMyType
    }
  }
]);

And type Column has been extended in the same way, to support Column<IMyType>.