[Fix] Trigger 'unlabeled' event when label is Deleted from PR (#34316)

This pull request updates the handling of issue label events in
workflows to distinguish between label additions and deletions,
introduces corresponding test cases, and extends the `IssuePayload`
structure to support this functionality.

### Enhancements to issue label event handling:
* Updated `matchIssuesEvent` in `modules/actions/workflows.go` to
differentiate between "labeled" and "unlabeled" events based on whether
labels were added or removed.
* Added a new field, `RemovedLabels`, to the `IssuePayload` struct in
`modules/structs/hook.go` to track labels that were removed during an
issue event.

### Testing improvements:
* Added `TestMatchIssuesEvent` in `modules/actions/workflows_test.go` to
cover scenarios such as label addition, label deletion, and label
clearing, ensuring the correct event type is triggered.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/structs/hook.go

@@ -418,6 +418,10 @@ type ChangesPayload struct {
 	Body *ChangesFromPayload `json:"body,omitempty"`
 	// Changes made to the reference
 	Ref *ChangesFromPayload `json:"ref,omitempty"`
+	// Changes made to the labels added
+	AddedLabels []*Label `json:"added_labels"`
+	// Changes made to the labels removed
+	RemovedLabels []*Label `json:"removed_labels"`
 }
 
 // PullRequestPayload represents a payload information of pull request event.