Add document Guide Mode

[Keavon]

Guide Mode (CtrlTab to enter and return to Design Mode) is used to draw shapes for the purpose of measurement, snapping, alignment, and constraints. Horizontal and vertical rulers are the simplest examples. Other shapes are also possible, though, and their outlines remain visible in Design Mode for visual and measuring reference.

Discussion about measurements and guide lines: https://discord.com/channels/731730685944922173/731730685944922176/814321942693543996 through https://discord.com/channels/731730685944922173/731730685944922176/814329010494242846

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This might actually end up being called Constraint Mode.