D.prefixed only emits its prefix on a line that starts at column 0.
When a BlockQuote is the first block of a list item, the item's
marker and the quote's content are joined onto the same line, so
the quote's Prefixed doc begins mid-line and its "| " bar is
dropped on that first line, while later wrapped lines (which do
start at column 0) render it correctly.
Force the blockquote onto its own line with D.cr before applying
the prefix. D.cr is a no-op when already at the start of a line,
so top-level and nested blockquotes are unaffected.
Fixes#11804
Gaurav Vijay Jadhav
b07038cd12
