add 'rootstock new-env <env-name>' command

[hholb]

Resolves: #19

This PR adds a new command rootstock new-env <env-name> that scaffolds an empty environment file making it easier to quickly define a new environment.

Screen.Recording.2026-03-20.at.9.54.50.AM.mov

I am open to other names if anything make sense or feels better than new-env. Things I thought about but didn't like any better:

• create-env
• create
• init-env

Other features:

• -o <output-file> to control where the output goes
• automatically appends _env if not given in the name
• if the environment already exists, it will not clobber it unless you pass --force

[WillEngler]

I dig it!