feat: 支持导出单次对话的全部提问记录为Markdown文件或HTML文件

[YoungZM339]

What this PR does / why we need it?

与 #471 有关，但未实现PDF导出

Summary of your change

使用marked和file-saver实现Markdown文件和HTML文件导出。 在实现PDF导出时，发现中文和特殊字符在导出PDF时会出现乱码问题。如果使用html2canvas后再导出PDF，则会导致文本无法复制。对于面向程序开发等场景的知识库来说，无法复制文本/代码是一个非常糟糕的问题，因此决定暂时不放出导出PDF的方法。

Please indicate you've done the following:

• [ Y ] Made sure tests are passing and test coverage is added if needed.
• [ Y ] Made sure commit message follow the rule of Conventional Commits specification.
• [ Y ] Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

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[shaohuzhang1]

ui/src/views/chat/pc/index.vue 文件冲突,无法merge

[YoungZM339]

收到，我晚上处理一下冲突再重启pr

[shaohuzhang1]

冲突已解决,已merge