Core: Add spoiler-only output mode

[Zannick]

What is this fixing or adding?

Adds a flag to Generate.py allowing for testing seed generation without multidata, which allows for manual confirmation that any e.g. performance-related changes to core or logic don't item placement (and with PYTHONHASHSEED set, playthrough and item paths) .

How was this tested?

Ran with --spoiler_only, and passed github workflow.