Render-blocking dark-theme initialization for dartdoc pages.

[isoos]

• A simpler alternative to fix: flash of white while loading page in dark mode on Firefox #8836 that focuses only on the rendering glitch.
• Uses the same initialization script and the same (blocking) way on dartdoc pages as the rest of pub.dev.
• dartdoc's own initalization does not support the OS-provided theme-preference yet https://github.com/dart-lang/dartdoc/blob/main/web/theme.dart - we need to implement that in dartdoc first (CSS and theme switches initialization should support prefers-color-scheme media queries. dartdoc#4069).

[isoos]

I have checked again with both Firefox and Chrome, with various different settings (including the <script blocking="render" ... attribute). The current init script improves the flashing on both browsers, and e.g. the blocking attribute doesn't add much on top of it. However, the site is not completely flash-free, in certain cases I've noticed a few more full or partial flashing, opening tracking issues for them (#8841 and #8842).