Introduce `kspOnlyVersion` to root build script

so that `kspVersion` can be constructed by

  `"$kotlinBaseVersion-$kspOnlyVersion"`

where `kotlinBaseVersion` is available in gradle.properties.
diff --git a/build.gradle.kts b/build.gradle.kts
index 88604bb..66bebe7 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -4,6 +4,12 @@
 val sonatypeUserName: String? by project
 val sonatypePassword: String? by project
 
+val kotlinBaseVersion: String? by project
+if (extra.has("kspOnlyVersion") && kotlinBaseVersion != null) {
+    val kspOnlyVersion = extra.get("kspOnlyVersion") as String
+    extra.set("kspVersion", "$kotlinBaseVersion-$kspOnlyVersion")
+}
+
 if (!extra.has("kspVersion")) {
     extra.set("kspVersion", "2.0.255-SNAPSHOT")
 }