feat: add quarto_install_extension() / quarto_use_template()

[mcanouil]

This PR brings two new wrapper functions: quarto_install_extension() and quarto_use_template()

Fixes #42
Fixes #27

[mcanouil]

Future: check if the current install/use function can use extension/format from within an R package (quarto-dev/quarto-cli#2079)
Although, this would require to set up a structure and document that structure, as for example the inst/rmarkdown/templates for Rmarkdown (https://bookdown.org/yihui/rmarkdown-cookbook/package-template.html).

[cderv]

Thanks @mcanouil for the PR. I tweaked quite a bit and use this opportunity to improve overall.