feat(variable service): add instances method
feat(variable service): add instances method

nagyistoce/camunda-bpm-sdk-js,camunda/camunda-bpm-sdk-js,nagyistoce/camunda-bpm-sdk-js,lightswitch05/camunda-bpm-sdk-js,lightswitch05/camunda-bpm-sdk-js,camunda/camunda-bpm-sdk-js