Add versioning extension point

[charphi]

Add versioning extension point to allow additional versioning schemes such as Calendar Versioning (aka CalVer).

[charphi]

Done in 82dc591

[koppor]

How to do CalVer? 😅

I have

## [2024-04-25]

[charphi]

Not possible yet but I have created an issue about that (see #253)

[charphi]

@koppor I've taken the time to think about it and 2024-04-25 is not a valid CalVer. Instead, it is a date used directly as a version.

I guess that's still a valid version, since the guidelines say that:

• Versions and sections should be linkable.
• The latest version comes first.
• The release date of each version is displayed.

The guidelines don't make it mandatory to have a linkable version followed by a hyphen and a date, even though all the examples show the opposite.

What do you think?

[koppor]

I checked https://calver.org/#youtube-dl - and it seems that a full ISO date can also serve as CalVer.

Yeah, good thing that "dash date" is optional 😅. Even though there should be a warning if it is not there.

Maybe, if CalVer != ISO Date, there is a problem reported for missing date?!

[charphi]

https://calver.org/#youtube-dl is not an ISO date since it used dot . to separate date components instead of dash -.