[Bug]: Missing quotes in case mapping key passed as a string

### Describe the issue:

We are using Slither Python API to analyze the expression of each node of function in a contract within Slither object.
The issue I need help with is that Slither expression does not differentiate between the following cases (in Solidity):
mapping(string => uint256) stringGeneBuckets;

```
//case # 1
stringGeneBuckets["myvariable"] = _geneId;
```

```
//case # 2
myvariable = "someStringValue"
stringGeneBuckets[myvariable] = _geneId;
```

For both cases mentioned above, the node.expression returned is `stringGeneBuckets[myvariable] = _geneId`, in the first case shouldn't it be `stringGeneBuckets["myvariable"] = _geneId`?

### Code example to reproduce the issue:

pragma solidity ^0.8.0;

contract GeneMapping {

// Mapping from a string to a uint256 value
mapping(string => uint256) public stringGeneBuckets;

function setGeneId(string memory _myvariable, uint256 _geneId) public {
stringGeneBuckets["myvariable"] = _geneId;
stringGeneBuckets[_myvariable] = _geneId;
}

// Function to retrieve a value from the mapping for a given key
function getGeneId(string memory key) public view returns (uint256) {
return stringGeneBuckets[key];
}
}

### Version:

0.9.6

### Relevant log output:

_No response_

Start by reproducing the contract example through Slither's Python API and inspect how node.expression represents the two mapping assignments. Trace the expression handling for string literals versus identifiers; done means the literal key remains distinguishable from the variable key in the returned expression.

Written by the indexing model from the issue text.