Feat/create in existing project

[thymikee]

Summary

Draft of create-app ran in existing project. Built on top of #29 as the logic for merging package.jsons is there.

Diff when in RNC CLI 0.77 project:

Auto-discovery of project name in initial prompt (we could reduce that to single question):

Test plan

No tests yet

[thymikee]

Tried this on Expensify codebase and it stalled after copying templates. Also revealing that there's a lot to cover. I wonder if it makes sense to throw Cali or something similar at this problem. In current state there's no point in overwriting native files other than build tools (to account for different cli command) or js tooling not essential to RNEF itself. Which means reusing the create-app strategy of copying templates is not really productive. For adding RNEF to existing projects we need a separate logic that will surgically add only necessary changes (dependencies, npm scripts, rnef.config, micro-adjustments in app.gradle and Podfile) and then ask user to git diff and adjust further if necessary.