Do not run Webpack bundle step when recompilation already required

[rose-m]

Right now the TypeScript compilation is separated into two different steps:

1. Run the TypeScript compiler via tsc
2. Bundle the generated .js files with Webpack

If during --watch mode a TypeScript file is modified both steps (1) and (2) are always executed again. However, while (1) is running and another source file change occurs, (2) is still executed and only after completion of (2) the compiler starts again with (1) followed by (2).

Goal: When the TypeScript compilation is currently running in step (1) and a source code change occurs, skip (2).