Fix ToMarkdown missing newlines for label-lists

[skipkayhil]

Ref rails/rails#50759

Previously, using ToMarkdown on a label-list would generate output that could not be reparsed by the RDoc::Markdown parser:

md = <<~MD
apple
: a red fruit

banana
: a yellow fruit
MD

doc = RDoc::Markdown.parse(md)
doc # => [doc: [list: NOTE [item: ["apple"]; [para: "a red fruit"]], [item: ["banana"]; [para: "a yellow fruit"]]]]

new_md = doc.accept(RDoc::Markup::ToMarkdown.new)
new_md # => "apple\n:   a red fruit\nbanana\n:   a yellow fruit\n\n"

new_doc = RDoc::Markdown.parse(new_md)
new_doc # => [doc: [list: NOTE [item: ["apple"]; [para: "a red fruit\nbanana\n: a yellow fruit"]]]]

The issue is that the PHP Markdown Extra spec requires a newline after each definition list item, but ToMarkdown was not putting newlines between label-list items.

This commit fixes the issue by properly appending a newline after each label-list item so that the output of ToMarkdown can be reparsed by RDoc::Markdown:

md = <<~MD
apple
: a red fruit

banana
: a yellow fruit
MD

doc = RDoc::Markdown.parse(mdoc)
doc # => [doc: [list: NOTE [item: ["apple"]; [para: "a red fruit"]], [item: ["banana"]; [para: "a yellow fruit"]]]]

new_md = doc.accept(RDoc::Markup::ToMarkdown.new)
new_md # => "apple\n:   a red fruit\n\nbanana\n:   a yellow fruit\n\n"

new_doc = RDoc::Markdown.parse(new_md)
new_doc # => [doc: [list: NOTE [item: ["apple"]; [para: "a red fruit"]], [item: ["banana"]; [para: "a yellow fruit"]]]]