chore(storybook): generate custom-elements.json for storybook argstable

[annawen1]

Closes #

This PR generates the custom-elements.json file needed to populate the ArgTypes table in storybook. The @element descriptor needs to be added to the JSDoc of the defined element which then gets picked up by custom-element-manifest/analyzer and generates the json file.

The yarn custom-elements script added to package.json and to the build command

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