[IMPROVEMENT] Improve the npm start script

[AbhinavBehal]

Description
The current npm start script executes the createjs and electron:serve commands in parallel. While this is fine once the .js files have been generated, new clones of the repository will not work correctly and might throw an error. As documented in #302, the npm start script should be changed to first generate the .js files, and then run the createjs and electron:serve commands.

Acceptance criteria

• As described above, the npm start should be changed to ensure that the generated .js files have been created before electron:serve is run.

Pointers / Solutions

• Could potentially move the required functionality into an external script.

Additional context
#302

Related issues
#269

[rmberriman]

Approved!

[lukethompsxn]

Approved, I have experienced the problems this causes when testing #302

[Z-Qi]

Approved as this should be the correct fix for the issues described in #302