feat: support to register markdown component

[mortalYoung]

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

• 新特性提交 / New feature

🔗 相关 Issue / Related Issue

💡 需求背景和解决方案 / Background or solution

• 支持编译时组件，即 markdown 组件
• 由于 [WIP]: feat: support to register markdown component #921 的改动较大，故拆分出此子 PR 以供 review

主要变更

• 新增 mdComponent 插件
• rehype 支持解析通过 dumi.registerMdComponent 该钩子注册的 markdown 组件
• 添加测试用例

📝 更新日志 / Changelog

Language	Changelog
🇺🇸 English	support to register markdown component
🇨🇳 Chinese	开放编译时组件能力

[github-actions]

🎊 PR Preview has been successfully built and deployed to https://dumi-preview-pr-933.surge.sh

[github-actions]

🎊 PR Mobile Preview has been successfully built and deployed to https://dumi-mobile-preview-pr-933.surge.sh

[codecov]

Codecov Report

Merging #933 (1a4887d) into master (74e48b3) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #933      +/-   ##
==========================================
+ Coverage   94.65%   94.70%   +0.04%     
==========================================
  Files         115      117       +2     
  Lines        2826     2851      +25     
  Branches      965      969       +4     
==========================================
+ Hits         2675     2700      +25     
  Misses        147      147              
  Partials        4        4              

Impacted Files	Coverage Δ
packages/preset-dumi/src/index.ts	100.00% <ø> (ø)
packages/preset-dumi/src/loader/index.ts	100.00% <ø> (ø)
...ckages/preset-dumi/src/transformer/remark/index.ts	100.00% <ø> (ø)
...es/preset-dumi/src/plugins/features/mdComponent.ts	100.00% <100.00%> (ø)
packages/preset-dumi/src/theme/loader.ts	96.20% <100.00%> (+0.09%)	⬆️
.../preset-dumi/src/transformer/remark/mdComponent.ts	100.00% <100.00%> (ø)
...ckages/preset-dumi/src/plugins/features/symlink.ts	90.90% <0.00%> (+0.90%)	⬆️

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[PeachScript]

👍，有空加下文档