Repeated occurence of `=` isnt handled correctly

### minimist doesn't handle correctly:
```
-a=b=c
{ _: [], a: 'b' }
```

### yargs-parser does handle correctly
```
-a=b=c
{ _: [], a: 'b=c' }
```

Spawned from: https://github.com/minimistjs/minimist/issues/2#issuecomment-1279851737