Adds greptile.json file for specifying triggers for Greptile on PRs (#4008)

Adds greptile.json file for specifying triggers for Greptile on PRs
This should avoid Greptile from getting triggered on every commit for a
PR.
It should still be able to get triggered manually by @ the bot.

Signed-off-by: Ammar <43151630+AmmarYAhmed@users.noreply.github.com>

Author: AmmarYAhmed

greptile.json

@@ -0,0 +1,3 @@
+{
+  "triggerOnUpdates": false
+}