Move build tasks to folder `./source/tasks`

Currently the build tasks are in the root of the repository in the folder `.build/tasks`. The problem with that is that getting code coverage visualization does not work as expected as the conversion task expects all source of the tested code to exist in the source folder (`src`, `source`, or `` under the project root.

The Azure code coverage are unable to find the source file to visualize the hits and misses as it already does with the rest of the code under the source-folder.

Suggest moving the tasks under the source folder and make ´build.ps1` to find the tasks there.