Gren 0.6 upgrade and misc improvements / refactoring

[blaix]

• upgrade to gren 0.6
• move cli/ to main project (in progress)
• Address TODO comments
• should it be possibe to run CLI with gren run? would require passing cli arguments.
• end result should still only require nodejs as the only dependency!

[blaix]

This issue title should probably change. Also not sure if this should be a hard requirement for v4...

[blaix]

Note: the 0.6 upgrade is complete on main.

[blaix]

Current thinking: I would like to continue having node be the main and only requirement for using prettynice, so I will still want an npm package, but I could have that built from the gren project repo instead of a separate project. Would remove some duplication with version numbers and things.