Use 'spotless' ktlin code linter and formatter

https://github.com/diffplug/spotless
https://github.com/diffplug/spotless/tree/master/plugin-gradle

Author: nicokosi

README.md

@@ -32,6 +32,10 @@ An executable file named `pullpitoK` should be generated in the current director
 
 ## Dev tasks
 
+### Format the code
+
+    ./gradlew spotlessApply
+
 ### Inspect the code
 
 Code inspection is configured on the master branch. See the [sonarcloud.io's dashboard](https://sonarcloud.io/dashboard?id=nicokosi_pullpitoK).

build.gradle.kts

@@ -4,6 +4,7 @@ plugins {
     id("com.github.ben-manes.versions") version "0.28.0"
     id("com.github.johnrengelman.shadow") version "5.2.0"
     id("com.adarshr.test-logger") version "2.0.0"
+    id("com.diffplug.gradle.spotless") version "3.25.0"
     application
 }