Move multiplatform target configuration into redwoodBuild DSL

[JakeWharton]

We still have a few projects that define the targets manually because they only need a subset. We should normalize these by the role of the project.

Currently we run all JS tests in the browser despite only a single module needing browser-based testing. We should default to NodeJS testing and allow overriding to use the browser only where needed. Don't think this is by role, but is perhaps a modifier on a role.

Maybe

fun kotlinMultiplatform(role: Role, vararg modifiers: RoleModifier)

enum class Role { RedwoodLibrary, TreehouseHostLibrary, TreehouseGuestLibrary, SampleApp, ... }
enum class RoleModifier { JsBrowserTests, ... }

[JakeWharton]

Actually we can probably infer kotlin-multiplatform from the role so perhaps it's just a single function or var.

[JakeWharton]

Gotta do this so I can link it from a blog post slamming convention plugins