Use swift-syntax to parse the slides manifest file

[eyeplum]

The current manifest parser is using Swift compiler's --driver-mode=swift to execute the manifest file (written in Swift) then dump the initialised TruffautSupport.Presentation as JSON in stdout.

This method was initially inspired by Swift Package Manager's ManifestLoader.

While Swift Package Manager's ManifestLoader was improved by making it sandboxed, Truffaut's manifest parser wasn't. This makes our current way of parsing both inefficient and insecure.

What's worse is that the way we distribute Truffaut.app and the TruffautSupport framework requires the framework and the running machine having the same version of the Swift compiler. This make it even harder to use.

Since now swift-syntax is available, we should move away from using the Swift compiler to execute the manifest file to parsing the manifest file literally.