Convert build script to Kotlin

- Add a new Node plugin. This automatically installs npm dependencies
   and provides a "NpxExecToDir" to dir task. This allows us to make the
   doc website task dependencies a little nicer, by simply chaining
   tasks together, rather than doing dependsOn + `input.files(the other
   task output)`.

 - Switch over to CurseForgeGradle from CurseGradle. The latter is
   super clunky to use in non-Groovy languages.

 - Copy our Modrinth description body to our repo, and add support for
   syncing it. We'll still have to do CF manually I think.

buildSrc/build.gradle.kts

@@ -23,5 +23,10 @@ gradlePlugin {
             id = "cc-tweaked.illuaminate"
             implementationClass = "cc.tweaked.gradle.IlluaminatePlugin"
         }
+
+        register("cc-tweaked.node") {
+            id = "cc-tweaked.node"
+            implementationClass = "cc.tweaked.gradle.NodePlugin"
+        }
     }
 }