Build: Add checkstyle plugin and add gradle task

build.gradle

@@ -2,6 +2,7 @@ plugins {
     id 'application'
     id 'java'
     id 'org.openjfx.javafxplugin' version '0.1.0'
+    id 'checkstyle'
 }
 
 group = 'ch.unibas.dmi.dbis'
@@ -43,6 +44,10 @@ test {
     useJUnitPlatform()
 }
 
+checkstyle {
+    toolVersion = '10.21.0'
+}
+
 tasks.named('jar', Jar) {
     manifest {
         attributes('Main-Class': application.mainClass.get())