Custom scalar type mapping does not work with relative paths on Windows

[DanielRose]

The example in the README for a custom scalar type mapping does not work on Windows.

module.exports = {
  endpoint: 'http://example.com/graphql',
  output: 'clients/myClient',
  options: { typeMapper: { location: 'path/to/typeMapper', types: ['Date'] } },
}

The problem is that the output and typeMapper's location are different, so a relative path will be generated (in relativeImportPath.ts). In this case, the relative path would be '../../path/to/typeMapper'

However, the code uses path.relative() to generate that path, which is OS-dependant: Node documentation. On Windows, this will generate the string '..\..\path\to\typeMapper', which is not a valid Node import string. Instead, use path.posix.relative() to force POSIX-compliant paths, even on Windows.