Support pasting more HTML elements

首先检查 html-to-markdown spike 以及链接的 Google Doc 测试用例。将现有的 paste-markdown 行为与列出的 HTML 元素进行比较，并为标题、代码、强调、列表和引用定义覆盖范围；完成的标准是这些剪贴板输入能够生成预期的 Markdown，同时不会使现有特殊情况发生回归。

This was originally an issue by @imjohnbo in github/special-projects, a private repository:

Summary

Today, content is pasted into GitHub comments (issues, PRs, discussions, etc) as plaintext, except for a few special cases managed by @github/paste-markdown.

It's a laborious process to add formatting and links of the copy source back into the paste target. For example:

• Copying/pasting part of a GitHub comment into a GitHub comment box – maybe in a different issue – in order to reply to it
• Copying/pasting part of a Google Doc shared with a customer into a GitHub comment box in order to add context to it
• ...

This issue would build on https://github.com/github/special-projects/issues/679 and investigate adding Slack-like paste functionality for the features GitHub developers care about, including:

name	html	markdown
headings	<h1>, <h2>...	#, ##, etc.
inline code	<code>	`code`
code block	<pre><code>	```code```
bold	<strong>	**bold**
italics	<em>	*italics*
ordered lists	<ol><li>	1. item
unordered lists	<ul><li>	- item
task lists	?	- [x] completed item
quotes	blockquote	> a long quote

Example

This Google Doc:

is pasted as:

Big Heading
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris eleifend consectetur nisl, vel vestibulum odio mattis sit amet. Integer vehicula rhoncus augue, eu congue magna porttitor eget.
Medium Heading
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris eleifend consectetur nisl, vel vestibulum odio mattis sit amet. Integer vehicula rhoncus augue, eu congue magna porttitor eget.
Small Heading
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris eleifend consectetur nisl, vel vestibulum odio mattis sit amet. Integer vehicula rhoncus augue, eu congue magna porttitor eget.

when I would like it to be:

# Big Heading
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris eleifend consectetur nisl, vel vestibulum odio mattis sit amet. Integer vehicula rhoncus augue, eu congue magna porttitor eget.
## Medium Heading
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris eleifend consectetur nisl, vel vestibulum odio mattis sit amet. Integer vehicula rhoncus augue, eu congue magna porttitor eget.
### Small Heading
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Mauris eleifend consectetur nisl, vel vestibulum odio mattis sit amet. Integer vehicula rhoncus augue, eu congue magna porttitor eget.

Spike

I started a spike over the weekend which allows for this:

Other

• Pasteboard Viewer helps understand what's in the macOS clipboard
• Here is a Google Doc with some test cases for convenience ❇️