Roslyn Source generators are not supposed to access I/O. To get around this we have to use `AdditionalFiles` to give the analyzer access to read/load the schema files.
To write the schema files we have to use a separate program altogether.
- [X] Adds SerializationSchemaGenerator
- [X] Splits out the SerializationGenerator code
- [X] Adds Enum migration rule
- [X] Adds legacy version (writing full int for version field)
- [X] Adds encoded int attribute
- [X] Adds intern string attribute
- [X] Fixes missing base deserialize/serialize