Implement OSV format

I think we should work toward the goal of supporting multiple formats including OSV: https://ossf.github.io/osv-schema/

For the API we should probably rename useStandardFormat to `useFormat`. This new property take a string with possible enumeration: `OSV` or `Standard`

```js
const vulnerabilities = await definition.getVulnerabilities(process.cwd(), {
  useFormat: "OSV"
});
```