Numerically-comparable equivalent of 'mpv-version' property

There is (AFAIK) currently no "proper" way for a script to check which version of mpv is running, as the mpv-version property seems mostly intended for display purposes. It would be nice to have another property providing the mpv version in a numerically-comparable format, similar to MPV_CLIENT_API_VERSION.

(An alternative would be to parse mpv-version myself, but I doubt its format is ever guaranteed to be parsable.)

Motivation: I'm the author of a plugin where I try to maintain compatibility with older mpv releases (back to 0.21.0 so far). While the presence/absence of new properties/commands is obviously easy to determine (although command-list itself is a recent feature), other changes may be more difficult to detect. For example, change-list didn't work with the file-local-options/ prefix until the introduction of shared-script-properties, so I check for that instead; this is not only hackish but also fragile, as it will break if that property is ever removed/renamed.

Start by locating the implementation and documentation for the mpv-version property and the MPV_CLIENT_API_VERSION constant. Define how the new property should encode versions for numeric comparison, then add the property and verify that scripts can distinguish releases such as 0.21.0; the issue names no specific test or file.