patch 8.2.2343: Vim9: return type of readfile() is any

Problem:    Vim9: return type of readfile() is any.
Solution:   Add readblob() so that readfile() can be expected to always
            return a list of strings. (closes #7671)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 59fa2b1..a891378 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1344,12 +1344,14 @@
 			ret_number,	    f_rand},
     {"range",		1, 3, FEARG_1,	    NULL,
 			ret_list_number,    f_range},
+    {"readblob",	1, 1, FEARG_1,	    NULL,
+			ret_blob,	    f_readblob},
     {"readdir",		1, 3, FEARG_1,	    NULL,
 			ret_list_string,    f_readdir},
     {"readdirex",	1, 3, FEARG_1,	    NULL,
 			ret_list_dict_any,  f_readdirex},
     {"readfile",	1, 3, FEARG_1,	    NULL,
-			ret_any,	    f_readfile},
+			ret_list_string,    f_readfile},
     {"reduce",		2, 3, FEARG_1,	    NULL,
 			ret_any,	    f_reduce},
     {"reg_executing",	0, 0, 0,	    NULL,