Release 3.1.5: remove debug println, drop worksheet leftover, modernize CI

- Remove stray println(child.value) from TreeNode.removeChild()
- Add regression test for removeChild() return value
- Delete leftover ExampleUnitTest.kt template test
- Remove Example.ws.kts worksheet and kotlin("script-runtime") from jvmMain
- Bump actions/checkout v2 -> v4 in CI workflows
- Update README install snippets to 3.1.5
- Bump version to 3.1.5

build.gradle.kts

@@ -13,7 +13,7 @@ plugins {
 
 val PUBLISH_GROUP_ID = "com.github.adriankuta"
 val PUBLISH_ARTIFACT_ID = "tree-structure"    // base artifact; KMP will add -jvm, -ios*, etc.
-val PUBLISH_VERSION = "3.1.4"
+val PUBLISH_VERSION = "3.1.5"
 
 val snapshot: String? by project
 
@@ -119,7 +119,7 @@ kotlin {
     sourceSets {
         val commonMain by getting
         val commonTest by getting { dependencies { implementation(kotlin("test")) } }
-        val jvmMain by getting { dependencies { implementation(kotlin("script-runtime")) } }
+        val jvmMain by getting
         val jvmTest by getting
         val jsMain by getting
         val jsTest by getting