Function relative

relative(base: string, target: string): null | string
Returns the relative path from base to source (case-insensitive), or null if not a subpath
Parameters
- base: string
  The base path
- target: string
  The target path
Returns null | string
The relative path, or null if not a subpath
Remarks
- Case-insensitive version of path/relative
Example
```
relative('Foo/Bar/Baz', 'foo/bar') // 'Baz'
```