set up convention plugins

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

buildSrc/src/main/kotlin/buildsrc/conventions/lang/kotlin-multiplatform-jvm.gradle.kts

@@ -0,0 +1,11 @@
+package buildsrc.conventions.lang
+
+plugins {
+    id("buildsrc.conventions.lang.kotlin-multiplatform-base")
+}
+
+kotlin {
+    jvm {
+        withJava()
+    }
+}