LibWeb: Refactor text justification code + only justify below threshold

All the justification-related code is now in
InlineFormattingContext::apply_justification_to_fragments and is
performed after all the line boxes have been added.

Text justification now only happens on the last line if the excess space
including whitespace is below a certain threshold. 10% seemed reasonable
since it prevents the "over-justification" of text. Note that fragments
in line boxes before the last one are always justified.

Userland/Libraries/LibWeb/Layout/InlineFormattingContext.h

@@ -36,6 +36,7 @@ public:
 
 private:
     void generate_line_boxes(LayoutMode);
+    void apply_justification_to_fragments(FormattingState::NodeState const& containing_block_state, LineBox&, bool is_last_line);
 };
 
 }