fix(webview): load Markdown custom styles in Firefox (#7913)

[yykaue]

Summary

• Fetch local webview stylesheets through the service worker in Firefox 149 and later.
• Apply the fetched CSS as constructed stylesheets while preserving the original base URL, media, and disabled state.
• Add regression coverage for custom Markdown preview styles.
• Update the changelog.

Validation

• npm run build
• npm run gulp compile-web
• npm run compile-client
• Manually verified with Firefox 151 that markdown.styles is applied correctly.

The targeted Playwright E2E test is currently blocked before the workbench loads by an unrelated CSS module MIME-type issue in the development build.

Fixes #7913

[code-asher]

Thank you for the PR!

This looks like something we ought to contribute to VS Code itself rather than maintaining as a patch here. Could we send this up to https://github.com/microsoft/vscode?

[yykaue]

Thank you for the PR!

This looks like something we ought to contribute to VS Code itself rather than maintaining as a patch here. Could we send this up to https://github.com/microsoft/vscode?

Thanks for the feedback! I completely agree — this is better suited to VS Code itself rather than being maintained as a patch here. I'm happy to send it upstream to https://github.com/microsoft/vscode.

I'll open a PR/issue there and link back to this discussion. Do you want me to close this PR in the meantime, or keep it open until the upstream change is merged?