Add Spotless conventions plugin

.gitignore

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+.gradle
+**/build/
+!**/src/**/build/
+
+# Ignore Gradle GUI config
+gradle-app.setting
+
+# Avoid ignoring Gradle wrapper jar file (.jar files are usually ignored)
+!gradle-wrapper.jar
+
+# Avoid ignore Gradle wrappper properties
+!gradle-wrapper.properties
+
+# Cache of project
+.gradletasknamecache
+
+# Eclipse Gradle plugin generated files
+# Eclipse Core
+.project
+# JDT-specific (Eclipse Java Development Tools)
+.classpath
+
+.kotlin

build.gradle.kts

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+plugins {
+    `kotlin-dsl`
+}
+
+repositories {
+    gradlePluginPortal()
+    mavenCentral()
+}
+
+dependencies {
+    implementation("com.diffplug.spotless:spotless-plugin-gradle:8.1.0")
+}

settings.gradle.kts

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+rootProject.name = "build-conventions"

src/main/kotlin/net.trueog.spotless-conventions.gradle.kts

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+plugins {
+    // Apply Spotless to the target project
+    id("com.diffplug.spotless")
+}
+
+spotless {
+    kotlin { ktfmt().kotlinlangStyle().configure { it.setMaxWidth(120) } }
+    kotlinGradle {
+        ktfmt().kotlinlangStyle().configure { it.setMaxWidth(120) } // JetBrains Kotlin formatting.
+        target("build.gradle.kts", "settings.gradle.kts") // Gradle files to format.
+    }
+}
+
+tasks.build { dependsOn(tasks.spotlessApply) }
+
+tasks.named("spotlessCheck") {
+    dependsOn("spotlessApply") // Run spotless before checking if spotless ran.
+}