fmt: respect single empty lines

Allow users to place empty lines between sibling elements.
Multiple empty lines in a row will be collapsed to a single empty line.

This allows for visual separation of content "blocks" while still
offering a normalized render of the document.

Author: kristoff-it

src/html/Ast.zig

@@ -744,7 +744,14 @@ pub fn render(ast: Ast, src: []const u8, w: *Writer) !void {
 
                     if (vertical) {
                         log.debug("adding a newline", .{});
-                        try w.writeAll("\n");
+                        const lines = std.mem.count(u8, maybe_ws, "\n");
+
+                        if (lines >= 2) {
+                            try w.writeAll("\n\n");
+                        } else {
+                            try w.writeAll("\n");
+                        }
+
                         for (0..indentation) |_| {
                             try w.writeAll("  ");
                         }
@@ -1296,6 +1303,7 @@ test "what" {
         \\    </a>
         \\  </body>
         \\</html>
+        \\
         \\<a href="#">foo</a>
         \\
     ;
@@ -1370,6 +1378,7 @@ test "self-closing tag complex example" {
     ;
     const expected =
         \\extend template="base.html"/>
+        \\
         \\<div id="content">
         \\  <svg viewBox="0 0 24 24">
         \\    <path d="M14.4,6H20V16H13L12.6,14H7V21H5V4H14L14.4,6M14,14H16V12H18V10H16V8H14V10L13,8V6H11V8H9V6H7V8H9V10H7V12H9V10H11V12H13V10L14,12V14M11,10V8H13V10H11M14,10H16V12H14V10Z"/>
@@ -1383,6 +1392,58 @@ test "self-closing tag complex example" {
     try std.testing.expectFmt(expected, "{f}", .{ast.formatter(case)});
 }
 
+test "respect empty lines" {
+    const case =
+        \\<div> a
+        \\</div>
+        \\
+        \\<div></div>
+        \\
+        \\<div></div>
+        \\<div></div>
+        \\
+        \\
+        \\<div></div>
+        \\
+        \\
+        \\
+        \\<div></div>
+        \\<div> a
+        \\</div>
+        \\
+        \\
+        \\
+        \\<div> a
+        \\</div>
+    ;
+    const expected =
+        \\<div>
+        \\  a
+        \\</div>
+        \\
+        \\<div></div>
+        \\
+        \\<div></div>
+        \\<div></div>
+        \\
+        \\<div></div>
+        \\
+        \\<div></div>
+        \\<div>
+        \\  a
+        \\</div>
+        \\
+        \\<div>
+        \\  a
+        \\</div>
+        \\
+    ;
+    const ast = try Ast.init(std.testing.allocator, case, .html, true);
+    defer ast.deinit(std.testing.allocator);
+
+    try std.testing.expectFmt(expected, "{f}", .{ast.formatter(case)});
+}
+
 pub const Cursor = struct {
     ast: Ast,
     idx: u32,