Add Directives to Variable definitions

[stevan]

This is a small commit to add directives to variable definitions. This is something which is supported in the JS reference implementation and has been added into the spec working draft. Please let me know if there are any issues with the PR.

Thanks, Stevan

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[stevan]

Adding some links to supporting information here:

An older RFC to add support for it
Now it is part of the current draft spec
And it is supported in the reference implementation