Front matter bracketed by "+++" should be detected

[chipotle]

Expected Behavior

Some blogging systems like Zola expect TOML front matter to have its first and last lines be +++, not ---, such as:

+++
title = "Dr. Neckbeard, or how I learned to stop worrying and love Emacs"
date = "2025-06-04 09:44:20"
+++

I expect this to be highlighted the same way TOML front matter would if the first and last lines were ---.

Actual Behavior

Markdown Mode doesn't recognize this as front matter, and so it is not highlighted appropriately, Flymake gets fussy about it (although that might well be a flymake-markdownlint problem), and so on.

Since Markdown Mode already recognizes TOML, this could probably be addressed simply by checking for the triple-plus signifiers at the same place the triple-dash signifiers are checked. (I know "this seems simple" is always dangerous in a feature request, though...)

Steps to Reproduce

Type front matter into the document. :)

Software Versions

Markdown mode: 2.8-alpha Emacs: 30.1 OS: macOS 15.5 (Sequoia)