Rollup merge of #148230 - fmease:rustdoc-highlight-tweaks, r=GuillaumeGomez

rustdoc: Properly highlight shebang, frontmatter & weak keywords in source code pages and code blocks

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<img width="517" height="532" alt="Screenshot 2025-10-28 at 23-21-02 pre rs - source" src="https://github.com/user-attachments/assets/5026761f-c604-4bcc-a699-9e75eb73dff6" /> | <img width="499" height="531" alt="Screenshot 2025-10-28 at 23-21-51 pre rs - source" src="https://github.com/user-attachments/assets/cc8c65e7-e3ad-4e20-a2c3-2623cf799093" />

Author: matthiaskrgr