Write data to a standard writable directory for cache data

[pacien]

Currently, the script attempts to write the user ID and the sponsorblock DB within the "sponsorblock_shared" directory. However, the directory in which the script is located isn't always writable (for example when the script is installed through a system package manager and used with mpv --scripts=[path to script]).

A more standard location would perhaps be $XDG_CACHE_HOME/mpv/scripts/sponsorblock/sponsorblock.{txt,db} on Linux. I'm not sure whether similar standard paths are defined on other platforms though.

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NixOS/nixpkgs#92507 (comment)

[po5]

PRs welcome, requirement is that the location should available on all systems mpv runs on.