Add support for publishing the fat jar

This commit adds compatibility with the Maven publish plugin, by
making sure that if the application is published, then the uber
jar is published as a classified artifact (`-uber`) in addition
to the regular jar.

A sample publishing configuration was added to the `app`, with
a "local" publishing repository in <rootproject>/build/repo.

For example, calling `./gradlew pAPTBR` will create the
`build/repo` directory with the corresponding Maven repository
structure after publishing.

app/build.gradle.kts

@@ -4,17 +4,25 @@ import org.gradle.api.tasks.testing.logging.TestLogEvent
 plugins {
     application
     id("com.uwyn.rife2")
+    `maven-publish`
 }
 
 base {
     archivesName.set("hello")
     version = 1.0
+    group = "com.example"
 }
 
 application {
     mainClass.set("hello.App")
 }
 
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(17))
+    }
+}
+
 repositories {
     // Use Maven Central for resolving dependencies.
     mavenCentral()
@@ -44,3 +52,17 @@ tasks {
         }
     }
 }
+
+publishing {
+    repositories {
+        maven {
+            name = "Build"
+            url = uri(rootProject.layout.buildDirectory.dir("repo"))
+        }
+    }
+    publications {
+        create<MavenPublication>("maven") {
+            from(components["java"])
+        }
+    }
+}