Allow using different tsconfig files for dev and build

[DorianGrey]

This PR adds the ability to use different configurations for dev and build mode for the typescript transpilation process.
Without further changes to the new tsconfig.build.json, the behavior will not change in any way, since it's just a no-op extension of the main tsconfig.json.

[wmonk]

Looks good! Do you think build is a clear enough name for this file? Possibly tsconfig.prod.json?

[DorianGrey]

Have to admit that I'm still thinking about this -

• build closely refers to the task it is used by
• prod primarily refers to the config version it is referenced by

So both makes sense. 🤔

[DorianGrey]

OK, decided to go with prod instead of build. Makes somewhat more sense in this case.

[mbrowne]

I was copying this PR into my app which was ejected, and I noticed that this line of webpack.config.prod.js was left unmodified:

new TsconfigPathsPlugin({ configFile: paths.appTsConfig })

I think maybe that should be changed to paths.appTsProdConfig also?

[DorianGrey]

Yes, there is already a PR for this change: https://github.com/wmonk/create-react-app-typescript/pull/323/files
It somehow got lost when creating and merging this PR. Just merged it,