Add scripts support to templates

[mrmckeb]

To test, add a "scripts" key to any template.json file, and use that template.

This code merges keys in, allowing template authors to add or replace the default scripts.

---

Example command below. While testing, you may need to bump the react-scripts version (via package.json) to 3.3.0.

node ./create-react-app/packages/create-react-app template-test \                                                                                                                                                                                                           feat/template-scripts
  --template file:./create-react-app/packages/cra-template-typescript/ \
  --scripts-version file:./create-react-app/packages/react-scripts/

[mrmckeb]

This code was previously lower.

[mrmckeb]

Replace any npm commands for Yarn users. This is the same logic we use for replacing those commands in the README file.

[iansu]

Do we do this for our scripts currently?

[ianschmitz]

Not sure that we need to be too smart about this.

[mrmckeb]

No, but I think this is a "nice to have" - as we're doing the same thing for the README in the same file.

It creates less confusion for people new to CRA too.

[mrmckeb]

I should clarify, we don't need this for our scripts - this is to ensure that scripts created by others are normalised - as we do with README files. This allows people to create scripts with commands like the below:

"scripts": {
  "build-dependency": "some-script",
  "compile": "npm run build-dependency && npm run build",
}

We should also consider (in future) normalising across operating systems too. For example, you can't do yarn build && yarn test on Windows (without WSL). But that's too much at this stage.

[ianschmitz]

npm run build-dependency && npm run build will work on windows without WSL. Is that what you meant?

[mrmckeb]

OK, that's good to know - I searched briefly and got mixed results. Thanks @ianschmitz.