Implement Swift 5 support

[tovkal]

This implements Swift 5 support.

Not sure if I have to ask for review from the Technical Committee nor how to do it.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

[wing328]

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09 @4brunu (2019/11)

[tovkal]

I have noticed there are still a couple of things that need to be done. I will continue working on this today.

[4brunu]

There is already similar PR #4086.
But it's not merged.
@tovkal can you please explain what changes did you introduced compared to Swift 4 generator?

[tovkal]

@4brunu Didn't see that PR. Nothing really, it also just copies the Swift 4 generator. Any reason why this PR has been stale for so many months?

I am closing this one.

[4brunu]

You can check the reasons for this in #4086.
But thanks for your PR 🙂 👍