Add `init` subcommand

[ncoghlan]

Writing venvstacks.toml from scratch is annoying, so it would be useful if there was a command that emitted a basic skeleton for subsequent editing.

This probably wouldn't even need tomlkit or jinja2 as a dependency - it could just use regular string formatting on a minimal template file.

[ncoghlan]

The workaround in the meantime is to use tests/sample_projects/venvstacks.toml as a starting point for writing a new stack definition.