Warn when attempting to animate a shader keyword

[AngriestSCV]

Shader keywords can not be animated the same way a float is. This PR issues a warning to the console when a shader keyword is being looked up for a material property.

[AngriestSCV]

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[SenkyDragon]

This method is called rapidly, including within the UI of the material editor. Printing to the debug log is probably not a good solution here. This would be more reasonable as a visible warning in the VRCFuryActionEditor, where it could warn if the property you've selected is a keyword.