Markdown: a Liquid tag alone on its line is reflowed into the neighbouring paragraph with proseWrap: always #19985

Prettier 3.9.6
Playground link

# Options (if any):
--parser markdown
--prose-wrap always
--print-width 120

Input:

{% callout type="info" %}
For the sake of example, the router's public address is 5.6.7.8 and its private address is 10.0.0.1.
{% /callout %}

Output:

{% callout type="info" %} For the sake of example, the router's public address is 5.6.7.8 and its private address is
10.0.0.1. {% /callout %}

Expected output:

{% callout type="info" %}
For the sake of example, the router's public address is 5.6.7.8 and its private address is
10.0.0.1.
{% /callout %}

Why?

Prettier has understood {% ... %} Liquid tags in Markdown since 1.13 (#3695): it parses them as an inline
liquidNode and never edits their contents. But it has no block form of that node. A tag that stands alone on a line
is, to the paragraph printer, the first word of the paragraph next to it, and proseWrap: always (and never) reflows
the two onto shared lines.

In every language that uses this syntax — Liquid in Jekyll ({% highlight %}, {% include %}, {% raw %}), Markdoc
({% callout %}, {% if %}), Twig — a tag alone on a line is a block tag and a tag inside a line of text is an inline
tag. The reflow changes which one it is. Markdoc, for example, renders the output above as an inline tag inside a <p>,
which puts an <aside> inside a paragraph. Two tags on adjacent lines fare worse
(playground):

We write the server first.
{% /if %}
{% if lang == "js" %}

becomes

We write the server first. {% /if %}
{% if lang == "js" %}

where the closing tag is now inline and the opening one is still a block, so they no longer pair, and Markdoc drops the
section. Because the result is Prettier's own output, prettier --check passes on it: the damage is silent in CI.

proseWrap: preserve (the default) is unaffected, since it keeps the author's line breaks. The problem appears exactly
when a project turns on wrapping, which is when people run --check in CI and rely on it.

Suggestion: treat a Liquid tag that occupies a whole line (leading whitespace allowed, nothing else on the line) as a
block-level node, the way a block html node is handled: print it verbatim, never fill it into a neighbouring
paragraph, and keep whether a blank line separated it from its neighbours. Inline tags, the current behaviour, are
untouched.

Workaround: a parser plugin that spreads the built-in Markdown parser and adds a preprocess step which puts a blank
line on each side of every standalone tag line (outside code fences) makes Prettier see each tag line as its own
paragraph. About forty lines; happy to share it or to turn the suggestion above into a PR if the maintainers want it.