feat (internal): support legacy model behaviors in SchemaRecord legacy mode

[runspired]

• id
• constructor.modelName
• currentState
• errors
• state flags
• object lifecycle flags
• serialize
• _createSnapshot
• reload
• changedAttributes
• rollbackAttributes
• unloadRecord
• deleteRecord
• save
• belongsTo references
• hasMany references
• destroyRecord

Tests

• id
• constructor.modelName
• currentState
• errors
• state flags
• object lifecycle flags
• serialize
• _createSnapshot
• reload
• changedAttributes
• rollbackAttributes
• unloadRecord
• deleteRecord
• save
• [-] belongsTo references (test once legacy belongsTo support is done)
• [-] hasMany references (test once legacy hasMany support is done)
• destroyRecord

TBD

• schema methods on instance
• schema methods on constructor

Note: only superficially tested. We're largely just copying model code over in a non-diverging manner so as long as the basic mechanics work we should be fine. So for instance we don't need to test every behavior around state flags.