Support trunk and dev snapshots

[fwal]

Trunk and dev releases should be supported.

Suggested api:

- uses: fwal/setup-swift@v1
  with:
    swift-version: "trunk"

and

- uses: fwal/setup-swift@v1
  with:
    swift-version: "5.3-dev"

[Jeehut]

Would love to see this! 👍