[services] Tunnelbroker - Add yarn scripts for dev-mode

Summary:
To run the tunnelbroker in dev-mode we need to export COMM_SERVICES_DEV_MODE environment variable.
Following the current code approach, add the corresponding commands for the tunnelbroker to package.json.

Linear task: ENG-1101

Test Plan: Run run-tunnelbroker-service-dev-mode and check that the tools::isDevMode() is true.

Reviewers: karol-bisztyga, palys-swm, ashoat

Reviewed By: karol-bisztyga, palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4126