Yarn Command support during playtest

[radiatoryang]

from #36

so this is also part of a bigger UX design problem with how Merino should handle Yarn Commands...

• if Merino executes Yarn Commands during edit mode, it would likely edit the current scene in harmful ways or just flat-out crash when that command isn't intended to execute outside of play mode
• but Yarn Command coroutines can be very important for timing dialogue, delays, setting variables or game state, etc.
• some users might engineer their game to support running in-game Yarn Commands during edit time

some possible design solutions:

• checkbox on PlaytestWindow ("Execute Commands?")
• user configurable blacklist / whitelist of Yarn Commands to execute and/or ignore

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most of the work will involve grabbing the code from the Yarn Spinner DialogueRunner (https://github.com/thesecretlab/YarnSpinner/blob/master/Unity/Assets/YarnSpinner/Code/DialogueRunner.cs) and porting it to Merino's internal dialogue runner (https://github.com/radiatoryang/merino/blob/master/Assets/Merino/Editor/Scripts/Playtest/MerinoPlaytestWindow.cs)... especially the DispatchCommand functionality

PRs are welcome, but if not, I'll get around to this in a month or so