Support setext headings in Markdown

[ExE-Boss]

Prettier 1.16.4

Playground link

{
	"parser": "markdown",
	"tabWidth": 4,
	"useTabs": true,

	/**
	 * `markdownHeadings` values:
	 * - `atx` (default)
	 * - `setext`
	 * - `setext-h1` (only `<h1>` uses the setext syntax)
	 */
	"markdownHeadings": "setext",
}

Input:

Setext Heading 1
================

Setext Heading 2
----------------

## ATX Heading 2

Output:

# Setext Heading 1

## Setext Heading 2

## ATX Heading 2

Expected behavior:

Setext headers are used:

Setext Heading 1
================

Setext Heading 2
----------------

ATX Heading 2
-------------

Preferably, I’d also like if there was an option to have only Heading 1 use the setext syntax, and the rest use the ATX syntax.

[brianary]

This is pretty important if you want the source Markdown/CommonMark/GFM text to be highly readable (a design goal of the language). ATX levels 1 & 2 don't exactly have much gravitas:

# Troubleshooting

## Setup

Lorem ipsum …

Sure, it's easier to parse, but that shouldn't be the driver of style.

Setext visually scans much more intuitively as a heading:

Troubleshooting
===============

Setup
-----

Lorem ipsum …

I'd advocate not just for the option, but for Setext to be the default header rendering.

[j-f1]

Looking at this again, I think an option would be unnecessary, but it would be OK if we preserved the style used in the original code.

[ExE-Boss]

Prettier currently preserves setext headings if they’re multi‑line, eg.:

Heading
[![Shields.io badge](https://img.shields.io/badge/Shields.io-badge-brightgreen)](https://shields.io)
=======

This is arguably worse than not preserving them at all, as it results in inconsistent formatting of headings.

[brianary]

@j-f1 That seems pretty reasonable. I wish it would clean up all the code Pretty forced its opinion on from my team members for the last year or so, though. It's going to be a lot of cleanup. Going along with whatever's there does seem at odds with having a standard or a strong opinion, too.

[Evpok]

If I read this and this correctly, there is not a lot of chance that preserving what is is there could be possible, since the parsers explicitly do not support keeping this information. So maybe allowing to configure the output style would be easier?

[brianary]

I've written what I'd consider an even-handed analysis of the available Markdown header styles that relates to this formatting question, whether opinionated or configurable.