Stop Markdown in doc attributes from leaking past the closing quotes

[robertoaloi]

When a -doc or -moduledoc attribute contains malformed Markdown, such as an unclosed code fence, the highlighting doesn't stop at the closing """. The rest of the module is highlighted as Markdown:

-doc """
Unclosed code fence:
```erlang
foo() -> ok.
""".
f() -> ok.   % highlighted as Markdown

Stop Markdown in doc attributes from leaking past the closing quotes

doc-directive included text.html.markdown directly between its begin and end patterns. The end pattern is only tried once the Markdown grammar hands control back, which it never does while a code fence is open. The include is now wrapped in a begin/while rule whose while pattern stops at the first line starting with """. The while pattern is checked at the start of every line, before the Markdown grammar runs, so the Markdown ends there whatever state the Markdown grammar is in. This is the approach suggested in microsoft/vscode-markdown-tm-grammar#175 (comment).

The fix has been in the ELP VS Code extension's copy of this grammar since March 2026 (WhatsApp/erlang-language-platform@ddeaba9b90). This PR brings it upstream.

Load a Markdown stand-in grammar in the tests

Until now, the tests had no coverage of doc-directive. They ran without a grammar for text.html.markdown, and when a rule's only include can't be found, vscode-textmate silently drops the rule. -doc and -moduledoc were therefore highlighted as generic directives, and the docstring and sigil snapshots recorded that.

This PR adds a minimal grammar for text.html.markdown under tests/grammars/, loads it with -g in both test commands, and regenerates the two snapshots. The only snapshot changes are doc attributes now being highlighted as meta.directive.doc.erlang, as they are in an editor that has the Markdown grammar.

Tests

tests/doc_markdown.erl covers an unclosed ``` fence in -doc and an unclosed ~~~ fence in -moduledoc. Without the fix, it fails: the closing """ and the next function head are highlighted as Markdown.

Co-authored with @bgsmeta.