feat: trigger watched task on new file

[radcool]

Currently, if a task is being watched and a file matching the source glob gets added, the task is NOT run; the file needs to be subsequently modified to trigger an actual task run. This PR changes this behavior so that the simple act of adding a file will trigger a task run.

This is based on the discussion and the preliminary code in https://github.com/go-task/task/issues/1179.