Make --include-extended-types default for Swift 5.9

[theMomax]

Bug/issue #, if applicable: apple/swift-docc#210

Summary

This PR makes --include-extended-types the default behavior for handling extended types when compiled with a Swift version greater or equal to 5.9. It also introduces the negative counterpart flag --exclude-extended-types, which is available from Swift 5.8. The default behavior for Swift 5.8 remains unchanged (equivalent to --exclude-extended-types).

The DocC documentation for the Plugin has been reworded.

The relevant part of the help text generated by the plugin's help flag has been reworded and shown both flags (--include-extended-types and --exclude-extended-types) along with the default value. The text is the same for Swift 5.8 and Swift 5.9, except for the default value.

Dependencies

None.

Testing

There are unit and integration tests making sure the behavior and help text is as expected for the Swift version the tests are run with. I ran the full bin/test suite with a Swift 5.7, Swift 5.8, and Swift 5.9 toolchain each and tested invocations manually with the same toolchains on a local test target.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [x] Added tests
• [x] Ran the ./bin/test script and it succeeded
• [x] Updated documentation if necessary

[theMomax]

@swift-ci please test

[theMomax]

@swift-ci please test