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-base.gradle.kts

@@ -0,0 +1,45 @@
+package buildsrc.conventions.lang
+
+import org.jetbrains.kotlin.gradle.targets.jvm.KotlinJvmTarget
+
+
+/**
+ * Base configuration for all Kotlin/Multiplatform conventions.
+ *
+ * This plugin does not enable any Kotlin target. To enable a target in a subproject, prefer applying specific Kotlin
+ * target convention plugins.
+ */
+
+plugins {
+    id("buildsrc.conventions.base")
+    kotlin("multiplatform")
+}
+
+
+kotlin {
+    jvmToolchain(11)
+
+    targets.configureEach {
+        compilations.configureEach {
+            kotlinOptions {
+                // nothin' yet
+            }
+        }
+    }
+
+    // configure all Kotlin/JVM Tests to use JUnit
+    targets.withType<KotlinJvmTarget>().configureEach {
+        testRuns.configureEach {
+            executionTask.configure {
+                useJUnitPlatform()
+            }
+        }
+    }
+
+    sourceSets.configureEach {
+        languageSettings {
+//         languageVersion =
+//         apiVersion =
+        }
+    }
+}