feat(ll)!: add new ll function and rename insert

ds_*ll_insert -> ds_*ll_append to better reflect what it does and there
is now ds_*ll_add to add data anywhere in the ll.

README.md

@@ -8,8 +8,8 @@ Example of working with a singly linked list (sll):
 char *a = "one";
 char *b = "two";
 ds_sll_t *ll = ds_sll_init();
-ds_sll_insert(ll, a); // [ "one" ]
-ds_sll_insert(ll, b); // [ "one", "two" ]
+ds_sll_append(ll, a); // [ "one" ]
+ds_sll_append(ll, b); // [ "one", "two" ]
 ds_ll_foreach(ds_sll_t, ll) {
     puts(cur->data);
     // one