codegen should error on unknown XML attributes in tags

[ab9rf]

I recently typoed typename for type-name in a structure definition. codegen silently accepted this erroneous definition instead of warning or erroring.

[lethosor]

Echoing Discord: this is hard because

• we don't have an exhaustive list of allowed attributes in codegen already
• some attributes have defaults (whether they should is another matter...)

maybe the answer is to do schema validation before ingesting files, if we can do that easily from perl.