Cannot distinguish between stdout and stderr in the `Exec` task

The `Exec` task can capture the output of the executed command by setting `ConsoleToMsBuild="true"` and reading the `ConsoleOutput` output parameter.

Unfortunately, the standard output and standard error streams are always combined: it's not currently possible to retrieve only the standard output for instance. This is an issue for commands that return the requested data in the standard output, but also write additional messages to the standard error.

One way to solve this would be to add additional output properties such as `StandardOutput`/`StandardError` (and maybe not trim those since the user probably wants the "raw" output when using them).