civil: add Compare method to Date, Time and DateTime

[shota3506]

Is your feature request related to a problem? Please describe.

This proposal suggests adding Compare method to civil.Date, civil.Time and civil.DateTime

Some structs in the standard package have Compare method which returns a integer value of either -1, +1, or 0.
https://pkg.go.dev/time#Time.Compare
https://pkg.go.dev/net/netip@master#Addr.Compare

It looks like the standard way to define ordered objects in Go (golang/go#50770 (comment)), and it is useful when we sort such objects by slices.SortFunc.

Describe the solution you'd like
I'll open a PR for this issue soon.

Describe alternatives you've considered
Of course, we can judge the order of civil objects by using exiting After and Before.
If there are opinions that it is unnecessary, I will respect them.

Additional context

[noahdietz]

Just reviewed the PR.

Thanks for the idea/contribution.

[noahdietz]

@shota3506 sorry, need to hold onto this for a little bit. We want to upgrade to Go 1.20 soon. Please reopen your PR after the reversion goes through and we will merge it as soon as we upgrade!

[shota3506]

@noahdietz Thank you for your review and comment!
I haven't checked go directive in this project sorry...
I'll wait for the version upgrade 👍

[shota3506]

@noahdietz

Hi ✋
The minimum version has been upgraded to 1.20, right?

I've reopened the PR #10193
Please check when you have time. Thanks.

[noahdietz]

Yes @shota3506 good eye :) Thanks!