Slither wont overwrite .sarif file

### Describe the desired feature

Slither should check before running its suite if there exists a .sarif file with the name defined for the current run, e.g.

```
touch latest.sarif
```

```
slither . --sarif latest.sarif
> INFO:Slither:latest.sarif exists already, the overwrite is prevented
```

Start at Slither's CLI handling for the --sarif option and reproduce the command with an existing latest.sarif file. The work is done when Slither reports that the named SARIF file already exists and prevents the run from overwriting it.

Written by the indexing model from the issue text.