Support link providers in Test Peek output window

[DanTup]

For the terminal, we can contribute link providers to detect URIs that VS Code cannot resolve, such as Dart's package:foo URIs:

However these are not available for Test Peek output like here:

It would be nice if we could support the same here so that stack traces that often show up here can be clicked, without having to use full absolute paths everywhere.

[connor4312]

This is a request for link providers in editors. We had a similar one before, which may even still be open, but I can't seem to find it.