Use standard environment variable for cache dir

[kohtala]

Hi. I'm using some packages using find-cache-dir and needed to have the cache out from node_modules. I did not find this mentioned, so I'd like to have it clarified.

Many systems already have a standard cache directory. It is specified at https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html. Namely one specified by $XDG_CACHE_HOME with default at $HOME/.cache

For Windows the folder should be under %LOCALAPPDATA%. Known folders on Windows are specified at https://docs.microsoft.com/en-us/windows/win32/shell/knownfolderid.

The "Finds the common standard cache directory" at top of readme.md does not really specify the scope of the cache directory. Common across packages in one project, projects of the user, or shared between users? If it is intended to be per project, the above locations can not be used without using some kind of project specific subfolder. If it is intended to work only for one project, I would like to see CACHE_DIR defined to point out that it should have different value for each project.