Add warning for constrainProportions in favor of targetAspectRatio

[gh-owongfigma]

With the release of Aspect Ratio Lock, we'll be supporting it in the plugin API via the targetAspectRatio property, set by lockAspectRatio and unlockAspectRatio. This should replace constrainProportions going forward. (The next update to the plugin API will already redirect sets of constrainProportions to modify targetAspectRatio).

Test Plan

• Tested manually by creating a plugin repository, and seeing the ESLint warning in VSCode.