Add output clearing to default renderer

[tommy-mitchell]

Closes #703.

I've only added it when using the default renderer with the output bar. Currently, setting task.output to an empty string clears the output:

task.output = ''

I've added an example as well. I'm not sure how to set up the snapshot tests with an ID.

Should I add output clearing elsewhere? (e.g. with another renderer or with the bottom bar, etc)

[cenk1cenk2]

Dear @tommy-mitchell,

Since empty strings happen at any point while logging a stream out, I would like to make that clearing intentional like maybe passing null to the task.output as you have did it.

But a more integrated way of doing this would be using https://github.com/listr2/listr2/blob/master/packages/listr2/src/lib/task-wrapper.ts which is the task you see as a user that is injected to all the tasks. This would emit an event on https://github.com/listr2/listr2/blob/master/packages/listr2/src/lib/task.ts where it can be listened to on the specific renderer that implements this. Instead of creating a function on the task, task-wrapper may directly emit the event since there is no specific function needed.

If this seems like a lot of changes I guess null is fine as a value to give to output. You just have to be sure that you clean the output bar as well as the bottom bar and update documentation as necassary.