Add method registerMergedStandardStreams in StreamInterceptor to merge
stdout and stderr so both can be intercepted in an stdout interceptor.
This will keep the relative order for both outputs, which makes it
easier to correlate error messages with the corresponding output.

Add new configuration parameter junit.platform.output.capture.merge to
merge stdout and stderr and publish it as STDOUT_REPORT_ENTRY_KEY to
all registered TestExecutionListener instances.

Issue: junit-team#3166

Add new CLI options --redirect-stdout and --redirect-stderr to
redirect stdout and stderr to a file, if both are set to the same file,
stdout and stderr will be merged so we can keep the relative order of
messages. This makes it easier to correlate error messages with the
corresponding output.

Issue: junit-team#3166

Document the changes for the previous two commit in the 5.11.0-M1
release notes and in the user guide.

Issue: junit-team#3166

Add @API annotation to new methods.

Issue: junit-team#3166