feat: add ft_strlen function

ft_strlen.c

@@ -0,0 +1,11 @@
+#include "libft.h"
+
+size_t	strlen(const char *s)
+{
+	size_t	i;
+
+	i = 0;
+	while (s[i] != '\0')
+		i++;
+	return (i);
+}

libft.h

@@ -1,6 +1,8 @@
 #ifndef LIBFT_A
 #define LIBFT_A
 
+# include <stdlib.h>
+
 /*
     The ft_isalpha() function tests for
     any character for which isupper(3) or
@@ -39,4 +41,10 @@ int	ft_isascii(int c);
 */
 int	ft_isprint(int c);
 
-#endif
+/*
+    The strlen() function computes the 
+    length of the string s.
+*/
+size_t	strlen(const char *s);
+
+#endif