Regex pattern to clear all problems of a task owner

Some compilers or build pipelines (e.g. create-react-app) emit different output if there are no errors vs. if there are 1+ errors or warnings. This makes it difficult to build problem matchers because start/end patterns need to have both a "errors" and a "no errors" version inside the same regex. It would be better if there were a new pattern in task config (e.g. `clearAllPattern`) that, if found, would clear all existing problems from the current owner. For non-background tasks this would be straightforward-- if the pattern is matched, then clear all problems from the owner and ignore all problems from the currently-executing task until the task exits. For background tasks, it'd be a little more complicated: 1) clear all problems from the owner 2) reset the state if it's currently after a `beginsPattern` but before an `endsPattern` 3) wait until the next `beginsPattern` before logging more problems. As a side effect, this feature would provide an escape hatch for users who want to clear problems by running a task (#50448), because you could create a task with a `clearAllPattern` that always matches.