version worlds, add an option to omit world template from saved state

[ssube]

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The current save command includes the world template in the save data, and load expects to find it there. That has a couple problems:

• the save file size increases, for every save file
• changes (bug fixes) to the world will not take effect in the saved state
• licensed content in the world template is duplicated in the state, making it harder to share

Sometimes fixing a save state to a particular version of a world can be desirable, especially if there are breaking changes. Semver provides a good solution to that: states can store the world template's version and warn if the major version changes.

If the available worlds are loaded when the game starts, that same list can be searched for the template later, on new game/load.