Provide a more detailed UniAST specification

**Is your feature request related to a problem? Please describe.**

我发现目前 ABCoder 实际上有三套用于生成 UniAST 的解析器：基于 Golang 的解析器、基于 LSP 协议的解析器，以及基于 TypeScript 的解析器。考虑到未来可能会引入更多的解析器，我们或许应该为 UniAST 给出一个更严格的定义。

I’ve noticed that ABCoder currently has three parsers for generating UniAST: a Golang-based parser, an LSP-based parser, and a TypeScript-based parser. Considering that more parsers may be introduced in the future, it might be necessary to provide a stricter definition of UniAST.

**Describe the solution you'd like**

重写 `docs/uniast-{zh|en}.md`，给出一个符合规范的 specification 文档，并且每一类 parser 都应该描述实现 `docs/uniast-{zh|en}.md` 的关键细节。

Rewrite `docs/uniast-{zh|en}.md`, and provide documentation for each parser on how it implements `docs/uniast-{zh|en}.md`.