Add options to validate WCAG 2.0 a11y requirements

[rnacken]

Is your feature request related to a problem? Please describe. I want PDFs that will pass an WCAG 2.0 a11y test (see https://www.w3.org/TR/2016/NOTE-WCAG20-TECHS-20161007/pdf.html)

When I try to validate the accessibility of a generated PDF with a tool like PAC 3 (https://www.access-for-all.ch/ch/pdf-werkstatt/pdf-accessibility-checker-pac.html), or http://checkers.eiii.eu/en/pdfcheck/ some issues come up that can nnot be taken care of with the current set of options provided by the @react-pdf API: Specifically what seems to be missing is:

• Bookmarks
• Natural language
• Missing metadata

Describe the solution you'd like

• I'd like to have the extra option for the Documents: Natural language
• I'd like the PDF to render extra metadata: XMP with the title, as well as PDF/UA identifier
• On Text component, the option of DisplayDocTitle
• I'd like to be able to add bookmarks (mentioned here already, but not in the context of a11y: https://github.com/diegomura/react-pdf/issues/237)

Furthermore, these other issues popped up while validating with PAC 3:

• "Link" annotation is not nested inside a "Link" structure element
• Link can not have an alternative description
• The document can't be marked as tagged, but this will probably mean a larger overhaul of components to control how you can navigate through the document with tool-assistance. Just marking the doc as 'tagged' will not be enough

Describe alternatives you've considered Currently, I believe a valid alternative to a11y-validated PDFs is to provide a link as the very first focusable element in the PDF that will link back to a webpage that does contain all the a11y features.

Additional context

[siren]

Natural language can be set like this:

<Document language="fi">

[rnacken]

@siren I don't know about that - from what I can tell in the source code, language does not seem to be a valid prop for Document:

Adding it also does not seem to change anything for the validation tools.