Improve image node

• Update image syntax so that if it is on a line by itself, it is treated as a block, otherwise treat it as an inline element.
  • The default schema would render block-level images in a figure instead of a p, but this behavior would be possible to override in a custom node definition’s transform function
• Update image syntax to allow passing additional attributes
  • E.g. ![alt](/href width=10 height=10 foo={arbitrary: "markdoc stuff"})
• Test that the image/link syntax works with GitHub flavored Markdown, or we have a story for that.

[mfix-stripe]

@nvanexan if you are using and Markdoc specific syntax (e.g. annotations), then those will not show up correctly in GitHub or iAWriter, until they support Markdoc syntax. If you are not, and you just use standard Markdown (e.g. ![foo](/url "title")), then it will still work in both all the contexts.

[samuelstroschein]

Are images nested in links part of this issue?

I just ran into the issue of using images with links. The following markdown throws { "id": "child-invalid", "level": "warning", "message": "Can't nest 'image' in 'link'" } but is valid on GitHub and a CommonMark reference implementation https://spec.commonmark.org/dingus/.

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