Create a Morphir Mill build integration plugin

[DamianReeves]

Feature Request

Create a Morphir Mill plugin that is tasked with wrapping the morphir-elm CLI and will also support direct morphir-scala tasks/commands.

Description of Problem:

Morphir is capable of using its morphir-elm make, morphir-elm gen, and morphir-elm test commands to generate code and run tests.

We have a need to generate Morphir IR and use morphir-elm's code generation capability within Scala builds.

We should create a Mill plugin that allows us to accomplish this.

Potential Solutions:

While we can bootstrap this process by doing Mill specific file and process creation, it should be noted we will want to accomplish the same behavior for abt and scala-cli (and possibly Gradle).