#### Support setext headings (#18473 by @fisker)

<!-- prettier-ignore -->
```md
<!-- Input -->
Setext Heading 1
================

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

Multiline
Setext
Heading
-------

<!-- Prettier stable -->
# Setext Heading 1

## Setext Heading 2

Multiline
Setext
Heading

---

<!-- Prettier main -->
Setext Heading 1
================

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

Multiline
Setext
Heading
-------
```
