Support ${} substitutions for all fields in defaults files

[wlupton]

Please see #7253 and #8024 for context.

The pandoc manual states that (in defaults files), the environment variable interpolation syntax only works in fields that expect file paths.

I would like to substitute values in some PDF engine options, but there's no way that pandoc can know which of these "expect file paths" so I'm wondering why the interpolation syntax can't simply be applied to all fields.

[jgm]

Certainly not all fields! A field might include some raw code (e.g. as part of a header-include) or a markdown code block.

We could think about PDF engine options, but there may be delicate issues there too. (Perhaps something like our interpolation syntax has a meaning for some options for some PDF engines?)

[wlupton]

Yes, I can see that expansion shouldn't be unconditional. Within a shell script it is unconditional, but people know that they have to take steps to avoid unwanted expansions.

So perhaps we need something like this (cf filters)?

pdf-engine-opts:
  - 'string-option-no-expansion'
  - expand: true
    value: ${USERDATA}/file.ext