patch 8.2.4820: not simple programmatic way to find a specific mapping

Problem:    Not simple programmatic way to find a specific mapping.
Solution:   Add getmappings(). (Ernie Rael, closes #10273)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f6d4b05..090f3a7 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1871,6 +1871,8 @@
 			ret_getline,	    f_getline},
     {"getloclist",	1, 2, 0,	    arg2_number_dict_any,
 			ret_list_or_dict_1, f_getloclist},
+    {"getmappings",		0, 0, 0,	    NULL,
+			ret_list_dict_any,  f_getmappings},
     {"getmarklist",	0, 1, FEARG_1,	    arg1_buffer,
 			ret_list_dict_any,  f_getmarklist},
     {"getmatches",	0, 1, 0,	    arg1_number,