Add game mode to Core.Flags

[chrisfls]

Took a little longer than what I've planned 'cause I tried doing it the hard way first, then I got lazy, then I did it the right way.

I'm not happy with the names I'm using, so I'm asking for name suggestions, please.

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This change is 

[PedroHLC]

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Reviewed 3 of 3 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[chrisfls]

Review status: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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src/Core/Flags.elm, line 7 at r1 (raw file):

    = HE1
    | HE2

@pedrohlc no suggestions here?

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Comments from Reviewable