Move codegen from Maven into the ftl tooling.

[alecthomas]

Use the same pattern as go-runtime/compile.Build, which uses scaffolder to generate stubs in Go. We want the same thing in Kotlin. Use internal.ScaffoldZip() rather than scaffolder directly, as it adds common functions.

The way we get the templates into the ftl binary differs between development mode and production. In development we load zip files directly from disk. In a production release build we use bit to build the zip and go:embed to embed it. See devel.go and release.go for examples.

I would just shove in the entire schema as the template context, removing any modules that the target module isn't importing.