Quarto: in LaTeX/PDF output, pass packages using the Lua API

[jdutant]

In Quarto users shouldn't specify LaTeX packages as (LaTeX commands in) header-includes. They should put them under the imagify key.

• Metadata syntax: make sure users can use the same keys under imagify as under format: pdf:
• Metadata coverage: make sure we pick up all documented Quarto keys that are relevant for the filter (provided we can access them when rendering to other formats than PDF)
• Pass to PDF output: pass packages specified under imagify to LaTeX when targeting LaTeX or PDF. In Pandoc, generate raw header-includes. In Quarto, use quarto.doc.include_text('in-header', text) and quarto.doc.use_latex_package(pkg, opt). See Quarto Lua API.