feat(extmarks): add sign name to extmark "details" array

Problem:  Unable to identify legacy signs when fetching extmarks with
          `nvim_buf_get_extmarks()`.
Solution: Add "sign_name" to the extmark detail array.

Add some misc. changes as follow-up to #25724

src/nvim/decoration.c

@@ -1022,6 +1022,10 @@ void decor_to_dict_legacy(Dictionary *dict, DecorInline decor, bool hl_name)
       PUT(*dict, "sign_text", CSTR_TO_OBJ(sh_sign.text.ptr));
     }
 
+    if (sh_sign.sign_name) {
+      PUT(*dict, "sign_name", CSTR_TO_OBJ(sh_sign.sign_name));
+    }
+
     // uncrustify:off
 
     struct { char *name; const int val; } hls[] = {