Generate constants for all Schema field names

[rnd256]

In OnStateChangeHandler in Unity, I almost always find myself checking if change.Field matches a constant string before deciding which sub-handler to run. String matching like this is error-prone because issues can't be detected at compile time. This could be resolved by either:

1. Providing OnChange support to individual fields rather than only on an object-level (I imagine this isn't possible across all Colyseus' platforms), or
2. Making schema-codegen generate constants for all the field names in the schema, so that they can be used instead of the developer writing out the constant strings themselves.