Suggest `std::path::MAIN_SEPARATOR_STR`

### What it does

Suggests `std::path::MAIN_SEPARATOR_STR` over `std::path::MAIN_SEPARATOR.to_string()`

### Lint Name

_No response_

### Category

_No response_

### Advantage

_No response_

### Drawbacks

_No response_

### Example

```rust
&std::path::MAIN_SEPARATOR.to_string()
```

Could be written as:

```rust
std::path::MAIN_SEPARATOR_STR
```
