restructure project to Kotlin Multiplatform

- custom POM location removed (there are now two POMs, one for the 'Kotlin Multiplatform' publication, and another for Kotlin/JVM, and more are on the way, which would lead to a cluttered build dir)
- renamed the directories (the directory name is how Kotlin Multiplatform chooses the published artifact ID, and there's no an easier way to change it.)
- updated README examples, and link to `-jvm` variant guide

settings.gradle.kts

@@ -23,6 +23,6 @@ dependencyResolutionManagement {
 enableFeaturePreview("TYPESAFE_PROJECT_ACCESSORS")
 
 include(
-  ":app",
-  ":lib",
+  ":urlencoder-app",
+  ":urlencoder-lib",
 )