Replace in-line usage of UUID generation for entity UUIDs with a helper function

[Josh Field]

We have a few different sources of UUID generation (MathUtils.generateUUID, uuidv4 etc) that result in different formats for UUIDs. We should standardize on the best source of randomness & uniqueness between them and add a helper function generateEntityUUID() that uses it and returns it typed as EntityUUID

[Daniel Belmes]

going with uuid package so we can remove some more dependencies on three.js :)