Preserve the blank lines typed after a list

The newline to break replacement only matched paragraph to paragraph, so a blank
line after a list was dropped and the text following it glued to the last item.
Only a following paragraph is matched, because without a blank line its text
would have continued the last item instead of starting a paragraph of its own.

UnitTests/Sources/AttributedStringBuilderTests.swift

@@ -684,6 +684,41 @@ struct AttributedStringBuilderTests {
         }
     }
     
+    /// A blank line the user typed after a list must survive, just like one between two paragraphs.
+    @Test
+    func newLineAfterList() throws {
+        let htmlString = "<p>Line 1</p>\n<ul>\n<li>Line 2</li>\n</ul>\n<p>Line 4</p>\n"
+        
+        let attributedString = try #require(attributedStringBuilder.fromHTML(htmlString), "Could not build the attributed string")
+        
+        #expect(String(attributedString.characters) == "Line 1\n  • Line 2\n\nLine 4")
+    }
+    
+    /// Without a blank line the trailing text is a lazy continuation of the item, so it stays on its line.
+    @Test
+    func noNewLineAfterList() throws {
+        let htmlString = "<ul><li>Line 1</li></ul><p>Line 2</p>"
+        
+        let attributedString = try #require(attributedStringBuilder.fromHTML(htmlString), "Could not build the attributed string")
+        
+        #expect(String(attributedString.characters) == "  • Line 1\nLine 2")
+    }
+    
+    /// A blank line is preserved wherever it can be told apart from mere block separation.
+    @Test
+    func newLinesBetweenBlocks() throws {
+        let expectations = ["<p>a</p>\n<p>b</p>": "a\n\nb",
+                            "<p>a</p>\n\n<p>b</p>": "a\n\n\nb",
+                            "<ol start=\"2\"><li>a</li></ol>\n<p>b</p>": "  2. a\n\nb",
+                            // A list can interrupt a paragraph, so this newline is block separation.
+                            "<p>a</p>\n<ul><li>b</li></ul>": "a\n  • b"]
+        
+        for (htmlString, expectation) in expectations {
+            let attributedString = try #require(attributedStringBuilder.fromHTML(htmlString), "Could not build the attributed string")
+            #expect(String(attributedString.characters) == expectation, "Wrong rendering for \(htmlString)")
+        }
+    }
+    
     @Test
     func unorderedList() throws {
         let htmlString = "<ul><li>1</li><li>2</li><li>3</li></ul>"