LibWeb: Add letter-spacing css property to Node

Author: https://github.com/kostyafarber
Commit: 537cbf55c3
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1911
Reviewed-by: https://github.com/AtkinsSJ ✅

Userland/Libraries/LibWeb/CSS/StyleProperties.h

@@ -116,6 +116,7 @@ public:
     Variant<LengthOrCalculated, NumberOrCalculated> tab_size() const;
     Optional<CSS::WhiteSpace> white_space() const;
     Optional<CSS::LengthOrCalculated> word_spacing() const;
+    Optional<LengthOrCalculated> letter_spacing() const;
     Optional<CSS::LineStyle> line_style(CSS::PropertyID) const;
     Optional<CSS::OutlineStyle> outline_style() const;
     Vector<CSS::TextDecorationLine> text_decoration_line() const;