patch 8.2.3510: changes are only detected with one second accuracy

Problem:    Changes are only detected with one second accuracy.
Solution:   Use the nanosecond time if possible.  (Leah Neukirchen,
            closes #8873, closes #8875)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 8a06f75..4abbdb9 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -5479,6 +5479,13 @@
 		0
 #endif
 		},
+	{"nanotime",
+#ifdef ST_MTIM_NSEC
+		1
+#else
+		0
+#endif
+	},
 	{"num64", 1},
 	{"ole",
 #ifdef FEAT_OLE