scala-agnostic launcher

[bjaglin]

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scalafix-cli is currently exposed as a coursier app, but the burden of chosing the right (full) scala version to load scalafix-cli (which is needed for ExplicitResultTypes ,which needs the presentation compiler), remains on the client.

To alleviate that, we could either

• Expose a new launcher abstracting the scalafix scala version logic currently only exposed for non-native JVM clients
• Make ExplicitResultTypes more portable by dynamically loading the Scala 2 PC via mtags in scalafix-cli_3 (see https://github.com/scalacenter/scalafix/pull/2023 for related discussions)

Originally discussed in the PR introducing support for scalafix into scala-cli