feat(build): opt into kotlin-assignment experiment

Newly introduced in 8.1: https://docs.gradle.org/8.1-rc-1/release-notes.html#kotlin-dsl-improvements:~:text=Experimental%20simple%20property%20assignment%20in%20Kotlin%20DSL%20scripts

We'll use this for the upcoming refactor of CrowdinPlugin

gradle.properties

@@ -12,6 +12,9 @@ org.gradle.vfs.watch=true
 # Enable experimental configuration caching
 org.gradle.unsafe.configuration-cache=true
 
+# Enable kotlin-assignment plugin
+systemProp.org.gradle.unsafe.kotlin.assignment=true
+
 # Enable Kotlin incremental compilation
 kotlin.incremental=true