LibHTML: Implement the <br> element for line breaking

The <br> element will produce a special LayoutBreak node in the layout
tree, which forces a break in the line layout whenever encountered.

This patch also makes LayoutBlock use the current line-height as the
minimum effective height for each line box. This ensures that having
multiple <br> elements in a row doesn't create 0-height line boxes.

Libraries/LibHTML/Layout/LayoutBreak.cpp

@@ -0,0 +1,17 @@
+#include <LibHTML/Layout/LayoutBlock.h>
+#include <LibHTML/Layout/LayoutBreak.h>
+
+LayoutBreak::LayoutBreak(const HTMLBRElement& element)
+    : LayoutNode(&element)
+{
+    set_inline(true);
+}
+
+LayoutBreak::~LayoutBreak()
+{
+}
+
+void LayoutBreak::split_into_lines(LayoutBlock& block)
+{
+    block.line_boxes().append(LineBox());
+}