feat: Option to add configuration to `package.json` rather than separate files

It would be nice if `gts` had the ability to add configuration to `package.json` rather than separate files. This can help users keep their top-level directories tidy. For example, the following files can be moved into `package.json`:

- `.eslintrc.json` -> `eslintConfig`
- `.eslintignore` -> `eslintIgnore`
- `.prettierrc.js` -> `prettier`

Here's an example: https://github.com/danielbankhead/picture-playlist/blob/37d3d9db31042afab63294c6b196865d25cecd27/package.json