Support ISO date strings

[isaacharrisholt]

Hello!

I ran into an issue recently where I wanted to parse some date strings (e.g. 2024-10-01) and assumed that birl.parse would handle them (that's on me - should've read the docs). It seems like there's no good way to get a Time (or Day) out of a value like this without some funky string shenanigans that seem a bit brittle.

Also, there doesn't seem to be a way to turn a Day into a Time without doing something like:

let assert Ok(time) = birl.parse("1970-01-01T00:00:00")
let time = birl.set_day(time, my_day)

It would be nice to be able to get a Time value from a date string (assuming 00:00:00 would be fine and pretty normal), so I suggest either:

• Add standalone date parsing to birl.parse (which might be a nightmare)
• Add a birl.parse_day function and a from_day function: fn(day: Day) -> Time

Happy to look into implementing these myself :)