feat: support demo.frontmatter.mobile

[CJY0208]

🤔 这个变动的性质是？/ What is the nature of this change?

• [x] 新特性提交 / New feature

🔗 相关 Issue / Related Issue

💡 需求背景和解决方案 / Background or solution

通过在 demo.FrontMatter 中增加 mobile 属性来控制不同 previewer block 对 theme-mobile 的启用情况

related: #876、#531

📝 更新日志 / Changelog

Language	Changelog
🇺🇸 English	support demo.FrontMatter.mobile
🇨🇳 Chinese	支持 demo.FrontMatter.mobile

[github-actions[bot]]

🎊 PR Preview has been successfully built and deployed to https://dumi-preview-pr-877.surge.sh

[github-actions[bot]]

🎊 PR Mobile Preview has been successfully built and deployed to https://dumi-mobile-preview-pr-877.surge.sh

[codecov[bot]]

Codecov Report

Merging #877 (142cc88) into master (cdd9635) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #877      +/-   ##
==========================================
+ Coverage   94.67%   94.69%   +0.02%     
==========================================
  Files         112      112              
  Lines        2608     2618      +10     
  Branches      861      865       +4     
==========================================
+ Hits         2469     2479      +10     
  Misses        135      135              
  Partials        4        4              

Impacted Files	Coverage Δ
packages/theme-mobile/src/builtins/Previewer.tsx	97.50% <100.00%> (+0.83%)	:arrow_up:

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[PeachScript]

❤️ 感谢贡献，dumi v1 不再更新，PR 将会关闭；dumi v2 已发布，欢迎继续参与 dumi 的建设，https://d.umijs.org/