fix: Support Typescript composite projects (#7147)

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