set up convention plugins

- tidy up build scripts to use convention plugins
- use centralised repo definition
- tidy up some of the build config

buildSrc/settings.gradle.kts

@@ -0,0 +1,16 @@
+rootProject.name = "buildSrc"
+
+pluginManagement {
+    repositories {
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}
+
+@Suppress("UnstableApiUsage")
+dependencyResolutionManagement {
+    repositories {
+        mavenCentral()
+        gradlePluginPortal()
+    }
+}