Provide a way to populate field values from Markdown/MDX processing pipeline

该 issue 未指定实现文件或测试；首先跟踪 Markdown/MDX 处理流水线，以及当前如何处理插件生成的 vFile.data。完成标准是将这些值与 frontmatter、code 和 _raw 一同暴露在生成的内容对象中，并为 searchMeta 示例提供覆盖。

Feature:
I'd like to use an MDX plugin to create programmatic meta (variable declarations) that can then be exposed in the final object contentlayer provides.

Use case:
An example of here this would be useful is generating a TOC, creating a list of unique keywords, etc.

Work around:
Use computed fields (which don't expose the the raw AST)

Related issues: https://github.com/kentcdodds/mdx-bundler/issues/169

Consider the following config structure

export default makeSource({
  contentDirPath: "content",
  documentTypes: [Posts, Things, People],
  mdx: {
    rehypePlugins: [rehypeSlug, rehypeAutolinkHeadings, searchMeta],
  },
});

Where searchMeta looks at paragraph nodes of mhast, grabs a list of unique words, and adds them to the metadata as searchMeta.

A markdown file with the structure of:

---
title: Hello World
slug: hello-world
---
Hello World! Please say Hello!

Would generate a final object of:

{
"title": "Hello World",
"slug": "hello-world",
"searchMeta": ["hello", "world", "please", "say"],
"code": "().....",
"_raw": "..."
}

For sake of complete, if not ugly code, here's a working example of the plugin that adds searchMeta to the data attribute of the vFile in the rehype plugin chain.

import { visit } from "unist-util-visit";

export default function searchMeta() {
  return (tree, file) => {
    visit(tree, { tagName: "p" }, (node) => {
      let words = node.children.reduce((collector, current) => {
        if (typeof current.value === "string") {
          let wordList = current.value
            .split(" ")
            .filter((word) => !word.includes(":"))
            .map((word) => word.toLowerCase().replace(/[^a-z0-9]/gi, ""))
            .filter((word) => word.length > 3);
          let newCollector = new Set([...wordList, ...collector]);
          return newCollector;
        } else {
          return collector;
        }
      }, new Set());

      file.data.searchMeta = [...words];
    });
  };
}