action-test-results-to-slack doesn't support workflow_dispatch

### Impacted plugin

[None / Other](https://github.com/Automattic/action-test-results-to-slack/tree/trunk)

### Quick summary

I just started using this plugin and noticed that all workflow_dispatch events will be grouped because the eventName is not handled.

I think it would be possibly sufficient to add:

```
if ( eventName === 'workflow_dispatch' ) {
msgId = `workflow_dispatch-${ Date.now() }`;
}
```
at: https://github.com/Automattic/action-test-results-to-slack/blob/trunk/src/message.js#L98

But, of course, another implementation would be fine! Just trying to avoid all workflow_dispatch being grouped as it is easy to lose track of those messages in the history.###

### Steps to reproduce

1. Run a workflow_dispatch workflow using the plugin
2. Make code changes
3. Run it a second time

### A clear and concise description of what you expected to happen.

Separate messages are sent to slack.

### What actually happened

All workflow_dispatch messages are grouped.

### Impact

One

### Available workarounds?

No and the platform is unusable

### If the above answer is "Yes...", outline the workaround.

_No response_

### Platform (Simple and/or Atomic)

_No response_

### Logs or notes

_No response_

Inspect src/message.js around line 98, focusing on how eventName determines message grouping. Run a workflow_dispatch workflow twice with a code change between runs and verify that Slack sends separate messages rather than grouping them.

Written by the indexing model from the issue text.