patch 8.0.0893: cannot get the scroll count of a terminal window

Problem:    Cannot get the scroll count of a terminal window.
Solution:   Add term_getscrolled().

src/evalfunc.c

@@ -835,6 +835,7 @@ static struct fst
     {"term_getcursor",	1, 1, f_term_getcursor},
     {"term_getjob",	1, 1, f_term_getjob},
     {"term_getline",	2, 2, f_term_getline},
+    {"term_getscrolled", 1, 1, f_term_getscrolled},
     {"term_getsize",	1, 1, f_term_getsize},
     {"term_getstatus",	1, 1, f_term_getstatus},
     {"term_gettitle",	1, 1, f_term_gettitle},