Implement API to cleanup unnecessary files

[unode]

Currently if a user keeps updating their ngless version they will accumulate binaries and resources in ~/.local/share/ngless for different versions.

This can be perceived as wasteful and requires that users understand this folder structure in order to know what can and cannot be removed.

Similarly to https://github.com/ngless-toolkit/ngless/issues/15 there could be an API (--cleanup) to delete old or unused resources given the ngless version in-use.