Generate Codable extensions adhering to `ExistentialAny`

[tomasharkema]

Swift 5.9 added ExistentialAny, which can be enabled by the swift setting: .enableUpcomingFeature("ExistentialAny"). The source that will be generated is backwards compatible in swift 5.9, in a codebase without ExistentialAny enabled.

I also took te liberty to run swiftformat, I hope it doesn't make the PR too cluttered. 🙈

I've added comments to the significant changes.

[soumyamahunt]

I also took te liberty to run swiftformat, I hope it doesn't make the PR too cluttered. 🙈

What command did you use to run swift-format? I don't see these changes when running swift-format.

The recommended way for running swift-format for this library would be to run:

npm run format

which runs:

swift run swift-format --in-place --ignore-unparsable-files --recursive Sources Tests

[soumyamahunt]

@tomasharkema I am planning to release all the pending fixes next week, let me know if you can resolve the comments and rebase by then.

[soumyamahunt]

superseded by #34.