ci: Add merge_group trigger to main workflow to enable merge queue (#6948)

## Explanation

This adds the `merge_group` trigger to the main workflow, so we can
enable the merge queue.

## References

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## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
- [ ] I've prepared draft pull requests for clients and consumer
packages to resolve any breaking changes

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Add `merge_group` trigger to `.github/workflows/main.yml` to enable CI
runs for the merge queue.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ffb79fe. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Mrtenz

.github/workflows/main.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches: [main]
   pull_request:
+  merge_group:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}