Allow properties in action.yml that are prefixed with an underscore

Describe the enhancement

I'm trying to automate creating documentation so in my action.yml I put a property on each outputs called example. This was then used to build the table seen here https://github.com/xt0rted/dotnet-sdk-updater#outputs.

The problem with this is the runner detects non-standard property names, logs an error for each of them, and then fails the run. To work around this I have a script that comments out the example lines before committing the release.

I'm wondering if it'd be possible to add an example property to outputs or better yet update the runner to allow properties prefixed with an underscore that way they can be used for other purposes.

Code Snippet

Additional information

My action.yml with additional properties https://github.com/xt0rted/dotnet-sdk-updater/blob/main/action.yml

Review the referenced action.yml and the update-readme.ts documentation automation first to understand the extra output property and its intended use. Trace the runner's action metadata validation, then confirm that underscore-prefixed properties are accepted without errors while standard output handling remains unchanged.