refactor: create a dedicated method for updating package.json

[G-Rath]

I'm aware of how much conflict this will create with #466 but oh well 🤷

[joshmcarthur]

I'm fine with this in principle, but we use template files a lot for similar use cases. I just wanted to prompt whether a template would be cleaner than doing a lot of file substitutions from code like this

[G-Rath]

We can't use templating because the file is touched by external code too i.e. react-rails and shakapacker write to it (and in fact we're depending on shakapacker:install to create the file initially).

Also I don't think it's as comparable: afaik all our templating files are generally within a single variant or maybe two whereas this particular file is modified by 5, and which are somewhat dependent on the actions of each other so I don't think templating would be as clean of a solution here.

[joshmcarthur]

Cool, that's all good. I thought it was worth raising, because it's quite easy to take an established pattern (like modifying the file in code), and use that over something that's already available via thor and the generator framework