Add an info subcommand

[ncoghlan]

Inspired by pdm info,
it would be useful to have a venvstacks info subcommand that reported:

• venvstacks, uv, and pbs-installer versions (just uv if Investigate dropping runtime PDM dependency #11 is implemented)
• location of venvstacks.toml
• default location of various folders (build, artifact output & metadata, export output & metadata)

And a --stacks option that dumped the layer information read from the stack specification and the environment lock files (essentially the same metadata that would be emitted for exported environments, but account for the fact the environments may not have been locked yet).

[Rugz007]

In this info command implementation, is it expected to save the last venvstacks.toml file path used by the user in the memory so that when the info command is used, it displays the last used file path?