fix: add label to issues and PR labeled/unlabeled events

When a workflow has

on:
  pull_request:
    types:
      - labeled
      - unlabeled

The payload misses the label field describing the added or removed
label.

The unlabeled event type was also incorrectly mapped to the labeled
event type.

modules/structs/hook.go

@@ -363,6 +363,7 @@ type IssuePayload struct {
 	Repository *Repository     `json:"repository"`
 	Sender     *User           `json:"sender"`
 	CommitID   string          `json:"commit_id"`
+	Label      *Label          `json:"label,omitempty"`
 }
 
 // JSONPayload encodes the IssuePayload to JSON, with an indentation of two spaces.
@@ -400,6 +401,7 @@ type PullRequestPayload struct {
 	Sender            *User           `json:"sender"`
 	CommitID          string          `json:"commit_id"`
 	Review            *ReviewPayload  `json:"review"`
+	Label             *Label          `json:"label,omitempty"`
 }
 
 // JSONPayload FIXME