Support pasting more HTML elements

Review the html-to-markdown spike and the linked Google Doc test cases first. Compare existing paste-markdown behavior with the listed HTML elements and define coverage for headings, code, emphasis, lists, and quotes; done means those clipboard inputs produce the intended Markdown without regressing existing special cases.

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 ❇️