Support plugin cdateFn type

[sya-ri]

cdateFn ignores plugins, so I can't use plugin features from TypeScript.

const cdateS = cdate().plugin(P => class extends P {
    subtract(diff: number, unit?: UnitForAdd) {
        return this.add(-diff, unit);
    }
}).cdateFn();

// cDateS: cdate.cdate