Support including hardcoded constants in the schema

[rnd256]

This is useful because it would be an easy way to share constants across all your Colyseus server and client platforms.

Examples of what this could be used for:

• The names of your Colyseus rooms
• A common version number
• Field names for JoinRoom options (although maybe this deserves a fully-shared class just like Schemas, rather than just shared field names)