TypeScript 3.0 introduced project references / composite projects. This allows to split a TypeScript build into sub-projects, while automatically keeping track of dependencies.
Currently react-scripts uses the legacy build TypeScript build mode without composite project support. Composite projects are supported for a long time by fork-ts-checker-webpack-plugin but wasn't enabled in the Webpack. This commit enables the build flag for TypeScript Webpack config, and fixes support for composite projects.
Closes #7147