Generate sourcemaps for easier debugging

[lannka]

Shall we enable sourcemaps for easier debugging?
I have a simple PR (#583) for that if there is no objections.

[Danielku15]

I have disabled source maps on purpose: In my development setup I use the browser development tools to debug and there I have terrible experience with source maps. They rather break proper debugging possibilities than they bring any value. Usually breakpoints do not work anymore and DevTools start to hang when you use source maps.

With the new ES6 compilation since 1.2 the final alphaTab.js contains clean and readable code. Mapping back any error location to the TypeScript codebase is usually a matter of seconds when searching for classes/methods in VsCode.