chore: add '.kotlin' directory entry in '.gitignore'

After KT-58223 implementation, the Kotlin gradle plugin stores project persistent
data inside '.kotlin'.

Author: Oleksandr Dzhychko

.gitignore

@@ -11,3 +11,4 @@ kotlin_gen
 /node_modules
 **/.ideaconfig
 **/.mpsconfig
+.kotlin