Add .kotlin/ to gitignore (facebook#48598)

Summary:
Adding .kotlin to gitignore. This folder starts to get used with K2 (with Kotlin 2.0) so we should be
adding it to the gitignore files

## Changelog:

[INTERNAL] - Add .kotlin to gitignore

Pull Request resolved: facebook#48598

Test Plan: N/A

Reviewed By: cipolleschi

Differential Revision: D68018000

Pulled By: cortinico

fbshipit-source-id: 78be3597071d07d105145d8ba94cd83cbf4f21bc

Author: cortinico

.gitignore

@@ -68,6 +68,7 @@ local.properties
 *.iml
 /packages/react-native/android/*
 !/packages/react-native/android/README.md
+.kotlin/
 
 # Node
 node_modules

packages/gradle-plugin/.gitignore

@@ -3,3 +3,4 @@ react-native-gradle-plugin/build/
 settings-plugin/build/
 shared/build/
 shared-testutil/build/
+.kotlin/

packages/helloworld/.gitignore

@@ -30,6 +30,7 @@ local.properties
 .cxx/
 *.keystore
 !debug.keystore
+.kotlin/
 
 # node.js
 node_modules/