🎆 Add static figure placeholder for images

[fwkoch]

This is the subset of the changes from #1701 which focuses on the static image field, an alternative to placeholder that will only be used in static exports, e.g. PDF. There is a little more discussion in that previous PR.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0cbc068

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
myst-directives	Patch
myst-transforms	Patch
myst-cli	Patch
myst-parser	Patch
myst-roles	Patch
myst-to-html	Patch
mystmd	Patch

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[agoose77]

@fwkoch I spoke with @stevejpurves about this today in #1157 (comment).

For this PR, the TL;DR is that it might be better to not add a new static field, and instead add an option that myst-theme should respect for ignoring placeholders (e.g. web-use-placeholder: false). Would you be against that? I can support the theme PR.

[rowanc1]

@fwkoch and I were talking about this in the context of another feature: only and I think that is both more flexible and hits this use case.

Let's explore that (which is necessary for supporting some of the books we are working on), and see if that replaces the need here.