Add devDependencies for custom templates

Created by: tomvalorsa

Is your proposal related to a problem?

Hey 👋good job on the recent release.

I’m really keen to make a custom template for CRA but want to add dev dependencies. Are there plans to add support for this is the near-ish future? Happy to throw up a PR if you’re busy with other stuff.

I made a fork of CRA a while ago so I could add custom templates but a community-supported effort is much more appealing!

Describe the solution you'd like

Simply adding another key to template.json for "devDependencies".

Where dependencies start to be installed here, there could be a function to wrap the reusable stuff (building up args, spawning process etc.) that could take an dependency type and the relevant slice of template.json.

Describe alternatives you've considered

Nothing at the moment as the template feature is obviously very new.

Additional context

This could potentially open up the possibility of adding peerDependencies too but I haven't really worked with them directly before, so not sure if this is useful/appropriate.