Setup Github Workflow to create the dev release of WPGraphQL Webhooks

[colinmurphy]

As discussed on stand up

• Should be re-usable for other plugins such as webhooks
• See Faust and Content Blocks

The outcome should be

1. When a PR is merged containing any code for Previews we should create a release for this
2. The release should be called dev-$COMMIT.zip for the time being as the plugin is still in alpha

[moonmeister]

closed by #232