Change WordBreakProperty underlying type to int8_t (#715)

This yields a ~1% performance improvements, likely
because the smaller types causes less load on memory
when bisearching the WordBreakProperty interval table

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/screen/string_internal.hpp

@@ -32,7 +32,7 @@ int GlyphCount(const std::string& input);
 
 // Properties from:
 // https://www.unicode.org/Public/UCD/latest/ucd/auxiliary/WordBreakProperty.txt
-enum class WordBreakProperty {
+enum class WordBreakProperty : int8_t {
   ALetter,
   CR,
   Double_Quote,