Add Placeholder component to utopia-api

[balazsbajorics]

This is important so that we can have an insertable Map primitive which has a Placeholder as mapped child.

Here's the actual component to use:

https://github.com/concrete-utopia/hydrogen-november/blob/ae55a172992f265b833bac8c83f13c496f358a11/app/components/Components.jsx#L16-L35