Drop direct use of LEVELS in R >= 4.5

There's no explicit encoding code for ASCII, so use charIsASCII()
("eapi", expected to appear in R-4.5.0).

Author: aitap

src/data.table.h

@@ -36,7 +36,11 @@
 /* we mean the encoding bits, not CE_NATIVE in a UTF-8 locale */
 #define IS_UTF8(x)  (getCharCE(x) == CE_UTF8)
 #define IS_LATIN(x) (getCharCE(x) == CE_LATIN1)
-#define IS_ASCII(x) (LEVELS(x) & 64) // API expected in R >= 4.5
+#if R_VERSION < R_Version(4, 5, 0)
+# define IS_ASCII(x) (LEVELS(x) & 64)
+#else
+# define IS_ASCII(x) (Rf_charIsASCII(x)) // no CE_ASCII
+#endif
 #define IS_TRUE(x)  (TYPEOF(x)==LGLSXP && LENGTH(x)==1 && LOGICAL(x)[0]==TRUE)
 #define IS_FALSE(x) (TYPEOF(x)==LGLSXP && LENGTH(x)==1 && LOGICAL(x)[0]==FALSE)
 #define IS_TRUE_OR_FALSE(x) (TYPEOF(x)==LGLSXP && LENGTH(x)==1 && LOGICAL(x)[0]!=NA_LOGICAL)