Define vendor extensions

[leonhard-s]

The current spec uses a slightly inconsistent mix of vendor (x-) fields that should be tidied up and documented.

Examples

• x-cast-to is not consistently placed and should always be next to the type field. Could also be renamed to x-type-cast-to or other variants whose intent is clearer

• x-reference-to should not contain the game namespace, only the fields. Another name could be considered too, though it is worth noting that these are not only used for primary keys

• x-deprecated or x-unused could be used to document fields that should not be exposed to users of generated code

• x-deprecated should point to alternatives