Add branches to merge-group schema

[lrotschy]

github/vscode-github-actions#40

Added branches to merge_group schema.

[muzimuzhi]

Just for the completeness, does merge_group trigger also support a branches-ignore filter?

Tests in muzimuzhi/hello-github-actions#9 show invalid filter won't fail a workflow run so the difference seems not observable by external users.

The blog introducing merge_group trigger contains a demo using branches filter, so branches is definitely supported. Since branches and branches-ignore are so closely related, I guess branches-ignore is supported as well.

[lrotschy]

I will open a new issue for adding branches-ignore to the schema.