#### Escape unindented indented pseudo setext headers (#19350 by @tats-u)

Indented lines composed of only `=` or `-` were previously unindented without being escaped, which caused them to be mistakenly parsed as setext headers. With this change, Prettier now escapes such lines to prevent the misinterpretation:

[Check parse results on CommonMark playground](https://spec.commonmark.org/dingus/?text=%3C!--%20Input%20--%3E%0ANot%20a%20header%0A%20%20%20%20%3D%3D%3D%0ANot%20a%20header%0A%20%20%20%20---%0ANot%20a%20list%20item%20or%20a%20header%0A%20%20%20%20-%0A%0A------%0A%0A%3C!--%20Prettier%20stable%20--%3E%0ANot%20a%20header%0A%3D%3D%3D%0ANot%20a%20header%0A---%0ANot%20a%20list%20item%20or%20a%20header%20-%0A%0A------%0A%0A%3C!--%20Prettier%20main%20--%3E%0ANot%20a%20header%0A%5C%3D%3D%3D%0ANot%20a%20header%0A%5C---%0ANot%20a%20list%20item%20or%20a%20header%0A%5C-)

<!-- prettier-ignore -->
```markdown
<!-- Input -->
Not a header
    ===
Not a header
    ---
Not a list item or a header
    -

<!-- Prettier stable -->
Not a header
===
Not a header
---
Not a list item or a header -

<!-- Prettier main -->
Not a header
\===
Not a header
\---
Not a list item or a header
\-
```
