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

[danielbankhead]

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