allow specification of package.main in template.json

Created by: EvanBoyle

Fixes https://github.com/facebook/create-react-app/issues/8373

I'm creating a template to work with pulumi. Pulumi programs need to have a main specified in the package.json in order to work (example). CRA currently blacklists/sanitizes main declarations which is preventing the generation of fully functional apps.

This change allows specification of the main field in template.json.

Testing was done by modifying the typescript template to include a main, and then running:

$ yarn create-react-app foo --template file:./packages/cra-template-typescript

which resulted in:

$ create-react-cloud evanboyle$ cat foo/package.json 
{
  "name": "foo",
  "version": "0.1.0",
  "private": true,
  "dependencies": {
    "@testing-library/jest-dom": "^4.2.4",
    "@testing-library/react": "^9.3.2",
    "@testing-library/user-event": "^7.1.2",
    "@types/jest": "^24.0.0",
    "@types/node": "^12.0.0",
    "@types/react": "^16.9.0",
    "@types/react-dom": "^16.9.0",
    "react": "^16.12.0",
    "react-dom": "^16.12.0",
    "react-scripts": "/Users/evanboyle/code/create-react-cloud/packages/react-scripts/react-scripts-3.4.0.tgz",
    "typescript": "~3.7.2"
  },
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject"
  },
  "eslintConfig": {
    "extends": "react-app"
  },
  "browserslist": {
    "production": [
      ">0.2%",
      "not dead",
      "not op_mini all"
    ],
    "development": [
      "last 1 chrome version",
      "last 1 firefox version",
      "last 1 safari version"
    ]
  },
  "main": "indes.js"
}