Publish allopen compiler plugin separately

- added also 'kotlin-allopen-compiler-plugin-embeddable' to be used with
  'kotlin-compiler-embeddable'
- 'kotlin-maven-allopen' now just depends on
  'kotlin-allopen-compiler-plugin' instead of "embeding" it into itself

^KT-52811 In Progress

plugins/allopen/build.gradle.kts

@@ -41,6 +41,8 @@ sourceSets {
     }
 }
 
+publish()
+
 runtimeJar()
 sourcesJar()
 javadocJar()