v3.24.0

1. feat: 增加“关闭窗口后保持运行”设置项
2. feat: 增加刷新预览快捷键 Ctrl/Cmd + R
3. feat: Mermaid 图形支持暗黑主题
4. feat: Mermaid 图形支持放大预览
5. upd: 导出文件名去除 .md
6. upd: 升级 PlantUML 版本
7. fix: 修复某些情况下 PlantUML 不能正确渲染问题
8. fix: 修复某些情况下不支持空行注释问题
9. chore: 增加 Ubuntu-18.04 构建产物

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1. feat: add "Keep running after closing window" setting item
2. feat: add refresh preview shortcut Ctrl/Cmd + R
3. feat: Mermaid graphics support dark theme
4. feat: Mermaid graphics support zoom in preview
5. upd: remove .md from export file name
6. upd: upgrade PlantUML version
7. fix: Fixed PlantUML not rendering correctly in some cases
8. fix: fix the problem that empty line comments are not supported in some cases
9. chore: add Ubuntu-18.04 artifact