notify adding reactions (#6072)

this PR adds an event for reactions received for one's own messages.

this will allow UIs to add notification for these reactions.

**Screenshots** at https://github.com/deltachat/deltachat-ios/pull/2331:

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Co-authored-by: iequidoo <117991069+iequidoo@users.noreply.github.com>

node/constants.js

@@ -50,6 +50,7 @@ module.exports = {
   DC_EVENT_IMEX_PROGRESS: 2051,
   DC_EVENT_INCOMING_MSG: 2005,
   DC_EVENT_INCOMING_MSG_BUNCH: 2006,
+  DC_EVENT_INCOMING_REACTION: 2002,
   DC_EVENT_INFO: 100,
   DC_EVENT_LOCATION_CHANGED: 2035,
   DC_EVENT_MSGS_CHANGED: 2000,