feat(add): Add npm packages to package.json if present

[nathanwhit]

Closes #25321

Ended up being a larger refactoring, since we're now juggling (potentially) two config files in the same add, instead of choosing one. I don't love the shape of the code, but I think it's good enough

Some smaller side improvements:

• deno remove supports jsonc
• deno install --dev will be a really simple change
• if deno remove removes the last import/dependency in the imports/dependencies/devDependencies field, it removes the field instead of leaving an empty object

[bartlomieju]

deno install --dev will be a really simple change

I don't see a test for that so I assume this will be done in a follow up?