Allow unions to have metadata

[parched]

Note: this doesn't do anything with the metadata,
but it's useful for sharing the schema with another tool that does use them.

It's unclear to how I should add a test for this.
It's not considered entirely valid because the attributes are unknown (I ignore these errors for my fbs files)

Checklist

• Updated CHANGELOG.md with relevant changes
• Added tests for any new/fixed functionality
• Added/updated documentation for new/changed code
• Checked that README.md still makes sense (and updated it if necessary)

[TethysSvensson]

I don't like just parsing and then throwing out the metadata. It at least needs to go into the cst, so the formatter can see it.

I also think it makes sense to validate it (and then reject it because we don't understand it).

[TethysSvensson]

Are you still interested in finishing this? :)

[parched]

I will get around to it when I have some spare time, but I'm more than happy if someone else wants to pick it up.