Add .code-with-filename CSS styles

[elnelson575]

Summary

Thanks @cpsievert for doing the hard bit in #244 . This just extracts it into a standalone like we talked about.

Adds styles for Quarto's code-with-filename feature — code blocks that display a filename label above the code:

• Rounded border with blue accent
• Filename label with blue background
• Removes inner border-radius so the code block fits flush inside the container
• Adjusts copy-button position for the nested layout

Test plan

• Code blocks with filename= attribute render with a labeled header and bordered container
• Copy button is correctly positioned inside code-with-filename blocks

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