LibWeb: Basic support for CSS text-indent: <length-percentage>

Note that this simple form of text-indent only affects the first line
of formatted content in each block.

Percentages are resolved against the width of the block.

Userland/Libraries/LibWeb/Layout/LineBuilder.h

@@ -57,6 +57,7 @@ private:
     CSSPixels m_available_width_for_current_line { 0 };
     CSSPixels m_current_y { 0 };
     CSSPixels m_max_height_on_current_line { 0 };
+    CSSPixels m_text_indent { 0 };
 
     bool m_last_line_needs_update { false };
 };