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>

CHANGELOG.md

@@ -35,6 +35,10 @@ current (development)
   See the [OSC 8 page](https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda).
   FTXUI support proposed by @aaleino in [#662](https://github.com/ArthurSonzogni/FTXUI/issues/662).
 
+### Screen
+- Breaking: `WordBreakProperty` becomes a uint8_t enum. This yields a 0.8%
+  performance improvement.
+
 
 ### Build
 - Check version compatibility when using cmake find_package()

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,