Add FQN for atomicfu plugin

[Prototik]

To be convenient with default Kotlin gradle plugin and Gradle's naming conventions for plugins I suggest to add full-qualified name for atomicfu plugin. It can be used in the following ways:

plugins {
  id("org.jetbrains.kotlin.jvm") version "${kotlin_version}"
  id("org.jetbrains.kotlin.plugin.atomicfu") version "${atomicfu_version}"
}

Or via Gradle Kotlin DSL extensions:

plugins {
  kotlin("jvm") version "${kotlin_version}"
  kotlin("plugin.atomicfu") version "${atomicfu_version}"
}

I choose the prefix org.jetbrains.kotlin.plugin similarly to org.jetbrains.kotlin.plugin.serialization from kotlinx.serialization

[elizarov]

Nice job. I think that the README should be adjusted correspondly, too.