Publish plugin marker artifacts to Maven Central

The spotless project already publishes the main artifacts to Maven Central, however it does not currently publish the plugin marker artifacts. This means that it's not possible to reference spotless like this:

```
pluginManagement {
plugins {
id("com.diffplug.spotless") version "6.3.0"
}
repositories {
mavenCentral()
}
}
```

Most users would likely still only use the Gradle Plugin Portal, so this is likely a minority and I could understand not wanting to implement. Personally, I've been trying to move away from the portal because the messy state of the repositories it proxies and the occassional outages stemming from legacy JCenter inclusion.