perf(@angular-devkit/build-angular): use worker pool for JavaScript transforms in esbuild builder

[clydin]

When using the experimental esbuild-based browser application builder, the JavaScript transformation steps of the build process will now be performed within a worker pool to allow for the steps to be executed in parallel when possible. This also moves the steps off of the main thread which provides more time for the build orchestration and esbuild integration code to execute.