add stylecheck and spotbugs

app/build.gradle.kts

@@ -8,6 +8,7 @@
 plugins {
   // Apply the application plugin to add support for building a CLI application in Java.
   application
+  checkstyle
   id("com.gradleup.shadow") version "9.0.0-beta12"
 }
 
@@ -16,6 +17,13 @@ repositories {
   mavenCentral()
 }
 
+sourceSets { main { java { srcDirs("src/main/java") } } }
+
+checkstyle {
+  toolVersion = "10.12.4"
+  configFile = file("./checkstyle.xml")
+}
+
 dependencies {
   // Use JUnit Jupiter for testing.
   testImplementation(libs.junit.jupiter)