Render Scribble margin-note as Markdown block-quote.

More precisely, do this for nested flows with the "refcontent" style.

For instance this Scribble:

    @margin-note{Note: This is a note. Let's make it long enough that the
    markdown output will have to line-wrap, to make sure the > mark starts
    each line properly.}

Will render as this Markdown:

    > Note: This is a note. Let's make it long enough that the markdown output
    > will have to line-wrap, to make sure the > mark starts each line
    > properly.

A site like GitHub.com will render this in a block-quote style
suitable for notes:

> Note: This is a note. Let's make it long enough that the markdown output
> will have to line-wrap, to make sure the > mark starts each line
> properly.

original commit: a3800cdc94d5f0c1b361d6e3ead6c1ebceb66288

collects/scribble/markdown-render.rkt

@@ -18,6 +18,7 @@
 
 (define table-ticks-depth (make-parameter 0))
 (define phrase-ticks-depth (make-parameter 0))
+(define note-depth (make-parameter 0))
 
 (define (render-mixin %)
   (class %
@@ -140,14 +141,19 @@
                  (render-flow d part ht #f)))))))
 
     (define/override (render-paragraph p part ri)
+      (define (write-note)
+        (write-string (make-string (note-depth) #\>))
+        (unless (zero? (note-depth))
+          (write-string " ")))
       (define o (open-output-string))
       (parameterize ([current-output-port o])
         (super render-paragraph p part ri))
       (define to-wrap (regexp-replace* #rx"\n" (get-output-string o) " "))
       (define lines (wrap-line (string-trim to-wrap) (- 72 (current-indent))))
+      (write-note)
       (write-string (car lines))
       (for ([line (in-list (cdr lines))])
-        (newline) (indent) (write-string line))
+        (newline) (indent) (write-note) (write-string line))
       (newline)
       null)
 
@@ -207,7 +213,12 @@
     (define/override (render-nested-flow i part ri starting-item?)
       (define s (nested-flow-style i))
       (unless (memq 'decorative (style-properties s))
-        (super render-nested-flow i part ri starting-item?)))
+        (define note? (equal? (style-name s) "refcontent"))
+        (when note?
+          (note-depth (add1 (note-depth))))
+        (begin0 (super render-nested-flow i part ri starting-item?)
+          (when note?
+            (note-depth (sub1 (note-depth)))))))
 
     (define/override (render-other i part ht)
       (cond

collects/tests/scribble/markdown-docs/example.md

@@ -43,3 +43,7 @@ The end.
 
 Returns a new mutable string of length `k` where each position in the
 string is initialized with the character `char`
+
+> Note: This is a note. Let’s make it long enough that the markdown output
+> will have to line-wrap, to make sure the > mark starts each line
+> properly.

collects/tests/scribble/markdown-docs/example.scrbl

@@ -52,3 +52,7 @@ Returns a new mutable string of length @racket[k] where each position in the
 string is initialized with the character @racket[char]
 
 }
+
+@margin-note{Note: This is a note. Let's make it long enough that the
+markdown output will have to line-wrap, to make sure the > mark starts
+each line properly.}