Added: sum & sumMany

Before now sum does the work of both single and multiple keys but this gets difficult when dealing with types and readability.

Old

// sum one key 
await Model.sum("fiat") // number
// sum multiple keys
await Model.sum(["fiat", "crypto"]) // {fiat: 200, crypto: 2.456}
// sum multiple fields with nick names 
await Model.sum({totalFiat: 'fiat', totalBitcoin: 'crypto'}) // {totalFiat: 200, totalBitcoin: 2.456}

New

Docs: https://xpresserjs.com/xpress-mongo/model.html#sum-async

// sum one key 
await Model.sum("fiat") // number
// sum multiple keys
const sum = await Model.sumMany(["fiat", "crypto"]) // {fiat: 200, crypto: 2.456}
// sum multiple fields with nick names 
const sum2 = await Model.sumMany({totalFiat: 'fiat', totalBitcoin: 'crypto'}) // {totalFiat: 200, totalBitcoin: 2.456}

With types support: sum and sum2 will be typed to have defined keys.