plugins/swift-format: Add swift-format plugin

JCWasmx86 requested to merge JCWasmx86/gnome-builder:swiftformat into main

This upstreams a plugin of mine.

Adds support for formatting Swift using swift-format. But it requires the swift code to be without parsing errors, as otherwise an empty string would be returned.

This plugin could have few bugs/leaks, as those async functions aren't my strength. It works on those paths: Everything fine, formatting fails, no executable found.

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