Escape setext heading underlines

### Version(s) affected

5.1.0

### Description

According to https://spec.commonmark.org/0.30/#setext-heading-underline a line containing any number of `=`s makes the line above it a heading 1.


### How to reproduce

HTML:
```
<p>Foo<br>=<br>Bar</p>
```

Output:
```
Foo  
=  
Bar
```

Expected output:
```
Foo  
\=  
Bar
```