Allow `caption` for code fences

[asymmetric]

Describe the feature you'd like to request

I'd like for fenced code blocks to support the caption attribute.

Right now if I want captions on code blocks, I have to use ```{code-block} $LANG` form, which has two issues:

• no syntax highlighting in most editors, since they don't know about this form
• the myst_number_code_blocks directive doesn't apply (this might be a separate bug)

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

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