function listHanModelPaths(lang, weight) {
  const pathInProject = 
    `build-files/models/shs-${lang.toLowerCase()}/${weight}`;
  const fullPath = expandPath(pathInProject);
  if (!fs.readdirSync(fullPath))
    throw Error(`Han models directory ${pathInProject} is not found.`);
  const result = fs.readdirSync(fullPath)
    .filter(f => f.endsWith('.json'))
    .map(f => path.join(pathInProject, f));
  return result;
}
Return a list of available Han models
@param { 'J' | 'K' | 'SC' | 'TC' } lang Language tag
@param { 'ExtraLight' | 'Light' | 'Normal' | 'Regular' | 'Medium' | 'Bold' 
| 'Heavy' } weight Model weight
@returns { string[] } Paths of model files relative to the project root