Increase default UNDO limit

[erkyrath]

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Currently it is hardwired to 10. Could be higher. Could also have a config preference (although this is not the same as a user-settable preference).

Forum thread: https://intfiction.org/t/inform-undo-limits-2024/72636

Before doing this, construct a rough measurement of how much space an UNDO record takes in a large game. (Counterfeit Monkey is our go-to example.)