auto complete dict key when possible

### Describe the Bug

two cases:

```py
d = {
    "1": 1,
    "2": 3,
}
d[""] <- here can give Literal["1"] or "2" as a candidate
```


```py
d = {}
d["1"] = 1
d["2"] = 5
d[""] <- same here
```


### Sandbox Link

_No response_

### (Only applicable for extension issues) IDE Information

_No response_