[json_syntax_generator] Class modifiers

[dcharkes]

We should investigate generating class modifiers for the generated syntax.

#2123 (comment)

Should all classes be final?

• Since [native_assets_cli] Make generated syntax files stand on their own #2102, the syntax files don't import each other. (We wouldn't want to export the syntax once they are separate packages.) But theoretically they could. So, I'm leaning towards no.

Should closed tagged unions be sealed?

• LinkMode is a closed tagged union, but this is not currently expressed in the JSON schema.
  • We'd need to massage the generated code so the super class is not instantiable.
• Asset is an inherently open tagged union. (After [native_assets_cli] Make generated syntax files stand on their own #2102 we're regenerating Asset in all syntax files.)

Thanks for the suggestion @HosseinYousefi!

Marking low prio as it's our own code internally to a package atm.