LibWeb: Add Layout::LineBuilder class for incremental line box layout

This class will be used to place items on lines incrementally instead of
the current two-phase approach.

Userland/Libraries/LibWeb/Layout/LineBuilder.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Layout/InlineFormattingContext.h>
+
+namespace Web::Layout {
+
+class LineBuilder {
+    AK_MAKE_NONCOPYABLE(LineBuilder);
+    AK_MAKE_NONMOVABLE(LineBuilder);
+
+public:
+    explicit LineBuilder(InlineFormattingContext&);
+    ~LineBuilder();
+
+    void break_line();
+    void begin_new_line();
+    void append_box(Box&);
+    void append_text_chunk(TextNode&, size_t offset_in_node, size_t length_in_node, float width, float height);
+
+    void break_if_needed(LayoutMode, float next_item_width);
+
+    float available_width_for_current_line() const { return m_available_width_for_current_line; }
+
+    void update_last_line();
+
+private:
+    InlineFormattingContext& m_context;
+    float m_available_width_for_current_line { 0 };
+    float m_current_y { 0 };
+    float m_max_height_on_current_line { 0 };
+};
+
+}