New callout support in Apex

Callouts just got a lot more flexible in the latest release of Apex. I now support Quarto, Pandoc, Obsidian/Bear, and Python Markdown flavors of callouts. Obsidian and Bear style still works The original blockquote-style syntax is still first-class: > [!NOTE] Heads up > This is a note. That still renders to Apex callout HTML with type-aware classes and optional collapsible behavior using + and -. Python Markdown callouts are now supported (behind a flag) If you want Python-style callouts, enable them explicitly with --py-callouts. !!! tip "Tip title" Tip body NOTE: Label-style note body >? NOTE: Collapsed label-style note > Body line for collapsed callout This mode is opt-in on purpose so normal Markdown content is not accidentally interpreted as callouts. Quarto callouts are also supported (behind a flag) Quarto-style fenced callouts are available with --quarto-callouts: ::: {.callout-tip} ## Tip title Tip body ::: One important caveat: Quarto callouts use the same ::: fence family…