add api to get the task-file from the compiler extension

[davidfirst]

This is one of the tasks in #2523.

For a compiler to declare its task file, the following API needs to be implemented:

const defineCompiler = () => ({ taskFile: 'task-file' });

This way, no need to enter the task filename to the compile configuration. It's enough to set the compiler name.

[AppVeyorBot]

❌ Build bit 1.0.10355 failed (commit 357672f940 by @davidfirst)

[AppVeyorBot]

❌ Build bit 1.0.10356 failed (commit 40f9ed9c3d by @davidfirst)