Add AllowedInInsets and AllowedInLayouts InsetLayout tags

Add AllowedInInsets and AllowedInLayouts InsetLayout tags

This allows to restrict the use of flex insets to specific contexts.

Note that this is currently very basis. It only considers immediately
containing insets/layouts and can be surpassed via cut and paste.

The use case that made me finally implement this are macros in covington
that make sense only in specific environments (but also consider \inst
in some article classes)