[gradle] Introduce gradle conventions using buildSrc

buildSrc/build.gradle.kts

@@ -0,0 +1,7 @@
+plugins {
+    `kotlin-dsl`
+}
+
+repositories {
+    gradlePluginPortal()
+}

buildSrc/settings.gradle.kts

@@ -0,0 +1 @@
+rootProject.name = "tweed5-conventions"

buildSrc/src/main/kotlin/de.siphalor.tweed5.local-runtime-only.gradle.kts

@@ -0,0 +1,6 @@
+plugins {
+    id("java")
+}
+
+val localRuntimeOnly = configurations.create("localRuntimeOnly")
+sourceSets.main.get().runtimeClasspath += localRuntimeOnly