Problem matchers: Support finding multiple matches in a single line

[lumaxis]

I'm trying to write a custom problem matcher for a tool that outputs all its results as a JSON object on a single line.
Today, Actions only supports finding one result per line. It would be great to be able to find multiple results per line, similar to the /g option for regexs.

It seems like the runner is using Regex.Match today which only yields one result max: https://github.com/actions/runner/blob/6c70d53eead402ba5d53676d6ed649a04e219c9b/src/Runner.Worker/IssueMatcher.cs#L68

I would suggest to look into maybe using Regex.Matches or something similar that enables multiple matches per line.