Enablement of DocC Extension Support as Default in Swift 5.9

After enabling the DocC Extension Support feature as opt in in Swift 5.8, the @swift-documentation-workgroup discussed enabling the feature by default in Swift 5.9 in the last WG meeting. The detailed meeting notes should become available here soon.

The WG voted to enable the feature by default. If this thread doesn't bring forward any major concerns, extension support will be enabled by default in the Swift DocC Plugin starting from plugin version 1.3, when the plugin is used with Swift 5.9 or higher. The behavior for Swift 5.8 remains unchanged, independently of the plugin version.

We only received positive feedback after enabling the feature as opt-in in Swift 5.8, so we feel confident to enable the feature by default now.

Starting from plugin version 1.3, the extension support feature can be disabled with the new flag --exclude-extended-types.

This change does not affect any other parts of the toolchain, as the default value is only toggled in the Swift DocC plugin. This also means people using Swift 5.9 won't see the default behavior change until they have also upgraded the Swift DocC plugin to version 1.3.

The PR implementing this change can be found here: Make --include-extended-types default for Swift 5.9 by theMomax · Pull Request #57 · apple/swift-docc-plugin · GitHub