Added description of core development priorities to Product Vision (#16647)
* Added description of core dev
* Moved new section to end
* Update product_vision.md
* Remove unneeded clause
* Update projectDocs/product_vision.md
* Update projectDocs/product_vision.md
Co-authored-by: Sean Budd <sean@nvaccess.org>
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Co-authored-by: Sean Budd <sean@nvaccess.org>