Feature request: Goals - Provision for Stepped Up Payments.

[Sohi819]

Current: Payment Per Period(PMT) is currently same through out the investment period.
Suggestion: Payment Per Period have a provsion to be stepped up by certain percentage(10% for eg.) yearly/monthly.
Changes: Monthly Investment needed, Goal Progress timeline & Goal Monthly Investment timeline

I was able to add step up rate in the Progress timeline by tweaking the project() function (SS👇🏼).
But I'm not able to calculate the PMT(solvePMTOrNper()) if step up rate is given. I tried:

1. Tweaking around financial lib.
2. Binary Search to match the target amount with low = 0, high = Non Step Up PMT.

Any help/suggestions would be appreciated.
Ref:

paisa/src/lib/goals.ts

Lines 58 to 96 in 26fb761

	export function project(
	fv: number,
	rate: number,
	targetDate: dayjs.Dayjs,
	pmt: number,
	pv: number
	): Forecast[] {
	rate = rate / (100 * 12);
	const today = now().startOf("month");
	if (fv <= pv) {
	return [];
	}
	if (targetDate.isSameOrBefore(today, "day")) {
	return [];
	}
	const nper = targetDate.diff(today, "months");
	if (nper <= 0) {
	return [];
	}
	const points: Forecast[] = [];
	let current = today.add(1, "month");
	while (current.isSameOrBefore(targetDate, "day")) {
	const value = financial.fv(rate, current.diff(today, "months"), -pmt, -pv, WHEN);
	points.push({ date: current, value, error: 0 });
	current = current.add(1, "month");
	}
	return points;
	}
	export function forecast(points: Point[], target: number, ARIMA: typeof Arima): Forecast[] {
	const configs = [
	{ p: 3, d: 0, q: 1, s: 0, verbose: false },
	{ p: 2, d: 0, q: 1, s: 0, verbose: false }
	];

https://github.com/lmammino/financial/blob/main/src/index.ts#L133-L149