[FR] include script in header could do clever insertion based on extension

Discussed with @andrewpbray in context of

• https://github.com/quarto-dev/quarto-cli/discussions/10654 with @andrewpbray

As a user, it seems natural to do

format:
  html:
    include-in-header:
      - file: mysidebar.js

to include a .js script in header of an HTML document

This won't work because file really expect only HTML content to be inserted directly. So a .html file content really.

In .pdf context, it would be .tex content, and in typst context a .typ file.

Maybe for HTML there is a way to be clever and write the HTML wrapping when .js or .css is detected as a file.

Regarding current situation, it says nothing, and does try to include the .js file content in the HTML document. Which creates an invalid HTML at the end, silently.

So we could also force .html extension check here when used format: html.

include-in-header can be technically as global option across format, but it seems format specific anyway, because raw content needs to be passed for the template.

Anyhow, registering the idea so that it is not lost in closed discussion.

Context of the discussion was Extension resource, and there is more to it than this option. Read the discussion for more.

Read the issue's include-in-header examples and the linked discussion, comparing the expected behavior for HTML, PDF, and Typst output. Define the format-specific handling or validation for file extensions, then verify that JavaScript input no longer silently produces invalid HTML and that existing raw-content behavior remains clear.