Allow (project) extensions to provide resources

[mcanouil]

Currently, Quarto extensions cannot really hold assets to be used by template or as default project setup.

For instance, images and files within a project extension cannot really be used:

• in template.qmd within the extension directory, you need _extensions/REPO/resource.txt, but when using the extension it becomes _extensions/ACCOUNT/REPO/resource.txt. (Leading to issue such as footer icons need HTML, which prevents extensions from resolving assets #9473)
• in a Quarto project extension, it's not possible to define default files. For example, you might want to create a Quarto project that has a Devcontainer setup, GitHub Actions workflows, etc., but this is currently not possible as beside format-resources no resources can be copied. (Include certain hidden files in templates (e.g. based on .quartoignore) #9351)

It would be nice for Quarto extensions to have a setting allowing users to define resources to be used by the project/formats.

There are two cases:

1. copy the resources listed at the root of the project on quarto use
2. add resources to "search path"

Possibly, the two could be controlled by users and used at the same time.