More declarative API for defining nodes and subprojects

[Deftu]

I've created the following mock-up for the in-settings DSL:

preprocess {
    create(rootProject) {
        // Now acts as a true root build file for the project passed to `create`.
        // This script no longer needs to handle nodes, as they're handled below.
        // OPTIONAL! Can be left unset
        rootBuildScript = "root.gradle.kts" // Relative to the project's directory

        // `root` sets that node as the root node, aka your "branch" node
        // this acts as the head from which all subsequent nodes will remap from
        //
        // `node` takes the subproject (auto-generated) name and the version number
        // as an integer representation, empty spaces 0-padded (aside from the major)
        root(node("1.21.8-neoforge", 1_21_08))
            // `to` links to the given node and returns it back, making you chain `to`
            // calls to link consecutive nodes. extra mappings will be configurable via
            // a sealed interface, either letting the plugin decide the filename for you
            // (while making you select path still), or giving you reign over the full path
            // (`EM` for short, as to keep lines readable... `"mappings"` here is the relative path)
            .to(node("1.21.8-fabric", 1_21_08, EM.named("mappings")))
            // of course, it must still be in correct order
            .to(node("1.21.7-fabric", 1_21_07) {
                // You can pass a block to `node` which lets you configure some things,
                // such as the `Project` it's tied to should you not want it to be done
                // auto-magically in the background
                include(":fabric-1.21.7") // Different name!
                project = project(":fabric-1.21.7")
            }).to(...) // And then right back to linking more nodes...
    }
}

This also entirely removes the requirement of any form of root.gradle.kts.