fixes #28 where making time struct with New() function did not counted leap year logic

[abtinokhovat]

New(time) function was not calculating correctly on days which were leap days or in leap years

`tl := time.Date(2025, 3, 20, 0, 0, 0, 0, time.Local)
ti2 := New(tl)
fmt.Println(ti2.Format("yyyy-MM-dd"))

ti := Date(1403, Esfand, 28, 0, 0, 0, 0, time.Local)
ti = ti.AddDate(0, 0, 2)

fmt.Println(ti.Format("yyyy-MM-dd"))

`

for example these days are the same how ever the calculation had errors both have to be 1403-12-30 but the one with ptime package is 1404-01-01

also this fixes this issue.

[yaa110]

@abtinokhovat Thank you for fixing the issue. Could you please add some unit tests for it as well?

[abtinokhovat]

Yep, of course, I will add some unit tests in near future.

[seyedmmousavi]

@abtinokhovat & @yaa110,
It is a good idea to document the calculation reference. Based on which method and mathematical formula have you done the calculations?

[abtinokhovat]

I'm now adding some documentation to the mathematic formula's I've used here and I will also add some tests here.

[abtinokhovat]

@seyedmmousavi @yaa110 I've added documentation as best as I could. The formula is somewhat ambiguous, and the result is based on my work on calendars from various sources over the years. I've included them as I remembered and had them. It is legacy code that I originally wrote in TypeScript and then rewrote in Go. I also added detailed test cases for each conversion function, both forward and backward.

[yaa110]

fixes #28