[r8] update .gitignore (#3882)

When I open xamarin-android in VS Code, the Java plugin creates
directories where it finds gradle projects.

Since r8 uses gradle, we should add to its `.gitignore`.

Author: jonathanpeppers

src/r8/.gitignore

@@ -1,3 +1,6 @@
 .idea/
 build/
-out/
+out/
+.classpath
+.project
+.settings/