ENH: Add a @pytask.mark.task decorator to mark functions as tasks irrespective of the name.

[tobiasraabe]

Is your feature request related to a problem?

Currently, task functions must be prefixed with task_ to be considered by pytask which is an unnecessary limitation as some user might prefer a different naming scheme.

For modules, the limitation is basically lifted since setting the configuration option task_files to * would basically collect all files.

Describe the solution you'd like

A decorator pytask.mark.task which optionally accepts a string which is the task id of the task.

@pytask.mark.task("real_task_id")
def do_this_and_only_this(depends_on, produces):
    ...
```

Then, there needs to be a small change in the collection module to also collect functions with the ``pytask.mark.task`` marker.