strict checking for format in condition content

[cderv]

This is a split from #4411 to handle the strick checking.

Currently when-format and unless-format works through format and aliases, and they could group formats. So there is no easy way to do strict checking.

Previous initial comment on this:

I completely sympathize with @mcanouil's here: we should have a "actually I really do mean this specific format" option. One immediate problem is that any changes to when-format will break backwards compatibility in a way that's hard to track and warn users.

I think the right solution is for us to come up with a consistent naming scheme for what we actually mean by format. The issue is that what goes into a "format" is actually a pretty complicated story. Here's a (probably incomplete) set of issues at play:

• Pandoc's format variants, such as html+emoji.

• Convenience under wildcards. We really often want an html wildcard that catches "stuff that a web browser reads". Examples of complications:

  • html5 is technically different from html, and so is revealjs.

  • epub is an "html" "format".

• quarto's custom formats: acm-pdf

• quarto's yml schemas depend on formats and format "groups". We should be consistent there, whenever it makes sense to be

• quarto tells pandoc useful, "harmless" lies about format: a pdf format declaration in a .qmd is actually sent to pandoc as latex; although pandoc supports pdf as an output format, quarto takes over the latex->pdf rendering part of the process.

I actually am now very reluctant to change this for 1.3.

Originally posted by @cscheid in #4411

See original post #4411 for more.