refactor CLI code

[nickswift]

Refactor pkl-gen-go.go to be more in line with idiomatic Go recommendations, and to lean more on the functionality provided by Cobra to reduce the complexity of this code.

This includes:

• Avoiding global state where possible
• Setting up resources in PreRunE and passing essential values to RunE via the command context.
• Delegating more of the argument handling and flag-binding logic to Cobra where possible.

This also touches on a similar area as #53, but the changes here are more comprehensive. I do think both changes can co-exist happily with a couple minor changes to this PR, but there should probably be some discussion first.

[nickswift]

This bit is where something more like what exists in #53 would be useful. Instead of eating the error with unwrapValue, we could have a constructor that creates a structure with all the CLI flag values in it that returns an error that we can handle properly.

This structure could also hold generator settings and the output path so that all we need to pass through the context is a reference to a single structure (as opposed to three separate values).