Add missing type enum for m.reaction (#1552)

changelogs/client_server/newsfragments/1552.clarification

@@ -0,0 +1 @@
+Fix missing `type` property in the JSON schema definition of the `m.reaction` event.  Contributed by @chebureki.

data/event-schemas/schema/m.reaction.yaml

@@ -37,3 +37,7 @@ properties:
               (see the [emoji variation sequences
               list](https://www.unicode.org/Public/UCD/latest/ucd/emoji/emoji-variation-sequences.txt)).
             example: "👍"
+  type:
+    enum:
+      - m.reaction
+    type: string