support {:L} with a ',' thousand separator

mysql-test/main/func_sformat.result

@@ -212,7 +212,7 @@ set names latin1;
 #
 select sformat('Num {:L}', 13800000000);
 sformat('Num {:L}', 13800000000)
-Num 13800000000
+Num 13,800,000,000
 select sformat('Num [{:20}]', 42);
 sformat('Num [{:20}]', 42)
 Num [                  42]

sql/item_strfunc.cc

@@ -56,7 +56,9 @@ C_MODE_END
 #include "sql_statistics.h"
 
 /* fmtlib include (https://fmt.dev/). */
-#include "fmt/format.h"
+#define FMT_STATIC_THOUSANDS_SEPARATOR ','
+#define FMT_HEADER_ONLY 1
+#include "fmt/format-inl.h"
 
 size_t username_char_length= USERNAME_CHAR_LENGTH;