Add XDG support for the cache dir

[RaafatTurki]

What is the problem this feature will solve?

the XDG spec is already implemented pretty nicely in this project, however I've noticed the cache dir ~/.bun/install/cache/ was neglected in that regard (I think it can be configure but having it work out of the box is better).

adding XDG support to the cache dir is a good idea because:

• makes wiping cache easier (deleting the cache dir will delete buns cache excluding its configs)
• unclutters the $HOME directory

What is the feature you are proposing to solve the problem?

Implementing this is pretty simple, instead of writing cache files into ~/.bun/install/cache/ a simple series of checks is performed to pick the right path with backward compatibility in mind ofc:

• if ~/.bun/ exists use it
• else if environment variable XDG_CACHE_HOME is set use $XDG_CACHE_HOME/bun/
• else use XDG_CACHE_HOMEs default value which is ~/.cache/bun/

If this gets implemented:

• Current users can move their ~/.bun/install/cache to XDG_CACHE_HOME (default is ~/.cache)
• New users will have to do nothing and it will just work as it should

What alternatives have you considered?

I don't think there is, Most of unix software supports this for a reason, I think bun should definitely have it.

[Electroid]

Duplicate of #1678