Publish 'kotlinx-serialization-compiler-plugin' separately

- Also publish 'kotlinx-serialization-compiler-plugin-embeddable' to be
used with 'kotlin-compiler-embeddable'.
- Now Serialization Maven and Gradle plugins do not embed plugin into
itself, but declare on published one normal dependency.
- 'kotlin-serialization-unshaded' publication is deprecated.

^KT-52811 In Progress

plugins/kotlinx-serialization/build.gradle.kts

@@ -72,6 +72,8 @@ sourceSets {
 
 optInToExperimentalCompilerApi()
 
+publish()
+
 runtimeJar()
 sourcesJar()
 javadocJar()