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.

gradle.properties

@@ -1,11 +1,11 @@
 org.gradle.jvmargs=-Xmx3G
+org.gradle.parallel=true
+
 kotlin.stdlib.default.dependency=false
+kotlin.jvm.target.validation.mode=error
 
 # Mod properties
-mod_version=1.100.10
+modVersion=1.100.10
 
-# Minecraft properties (update mods.toml when changing)
-mc_version=1.16.5
-mapping_version=2021.08.08
-forge_version=36.2.34
-# NO SERIOUSLY, UPDATE mods.toml WHEN CHANGING
+# Minecraft properties: We want to configure this here so we can read it in settings.gradle
+mcVersion=1.16.5