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

buildSrc/src/main/kotlin/buildsrc/conventions/lang/kotlin-jvm.gradle.kts

@@ -1,47 +0,0 @@
-package buildsrc.conventions.lang
-
-import buildsrc.utils.Rife2TestListener
-import org.gradle.api.tasks.testing.logging.TestExceptionFormat
-import org.gradle.api.tasks.testing.logging.TestLogEvent
-import org.jetbrains.kotlin.gradle.dsl.JvmTarget
-import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
-
-/**
- * Common configuration for Kotlin/JVM projects
- *
- * (this can be removed after Kotlin Multiplatform migration)
- */
-
-plugins {
-    id("buildsrc.conventions.base")
-    kotlin("jvm")
-    id("io.gitlab.arturbosch.detekt")
-    id("org.jetbrains.kotlinx.kover")
-}
-
-java {
-    withSourcesJar()
-}
-
-kotlin {
-    jvmToolchain(11)
-}
-
-tasks.withType<KotlinCompile>().configureEach {
-    compilerOptions {
-        jvmTarget.set(JvmTarget.JVM_11)
-    }
-}
-
-tasks.withType<Test>().configureEach {
-//    useJUnitPlatform()
-
-    val testsBadgeApiKey = providers.gradleProperty("testsBadgeApiKey")
-    addTestListener(Rife2TestListener(testsBadgeApiKey))
-    testLogging {
-        exceptionFormat = TestExceptionFormat.FULL
-        events = setOf(TestLogEvent.PASSED, TestLogEvent.SKIPPED, TestLogEvent.FAILED)
-    }
-
-    finalizedBy(tasks.matching { it.name == "koverXmlReport" })
-}

buildSrc/src/main/kotlin/buildsrc/conventions/publishing.gradle.kts

@@ -89,6 +89,7 @@ signing {
         !isSnapshotVersion() || gradle.taskGraph.hasTask("publish")
     })
 }
+
 tasks.withType<Sign>().configureEach {
     val signingRequiredPredicate = provider { signing.isRequired }
     onlyIf { signingRequiredPredicate.get() }
@@ -106,3 +107,9 @@ val javadocJar by tasks.registering(Jar::class) {
     from(tasks.dokkaJavadoc)
     archiveClassifier.set("javadoc")
 }
+
+publishing {
+    publications.withType<MavenPublication>().configureEach {
+        artifact(javadocJar)
+    }
+}