[3.x] Backport the Time singleton from master

[aaronfranke]

This PR backports the Time singleton from #49123. However, unlike that PR, this PR doesn't change the methods in the OS (_OS) singleton or use these methods internally, for the sake of preserving compatibility with 3.x projects.

This has the following use cases:

• If someone needs time features not available in OS, they can use the Time singleton.
• If someone is currently using the OS methods, they can switch to the Time singleton to future-proof their projects.

This was discussed with @ChronicallySerious on RocketChat, since he could make use of this class in the version control integration in Godot 3.5.

[twaritwaikar]

Thanks for this! I was able to workaround the need for the Time class by just passing on the responsibility (of converting timestamps and the offsets to human readable strings) to the underlying VCS plugin.

But after this is merged, I can update the VCS interface to just require a timestamp and a timezone offset from the plugin and Godot handles the rest \o/

[qarmin]

I will change test project around 17-19.
This change broke test project timer, so it can't close self after 30s
Nope, I didn't read this properly, leak is not caused by test project, but problem with code

[akien-mga]

You could also include a cherry-pick of #54256 in this PR once the master one is merged.

[akien-mga]

Maybe add references to the Time singleton in the relevant OS methods which it's duplicating and deprecating?

[akien-mga]

Thanks!