Improve union types of the modelling comands

[Irev-Dev]

They probably need to be broken now, here's an exerpt from models.ts

xport type ModelingCmd_type =
  | 'StartPath'
  | {
      MovePathPen: {
        path: ModelingCmdId_type /* The ID of the command which created the path. */;
        to: Point3d_type /* Where the path's pen should be. */;
      };
    }
    // ...

the union of a string and an object means that user's can't be more specific like

type MoreSpecificType = ModelingCmd_type['MovePathPen'] // not possible

Here are some hacks we're already using in the app to get around this

type SketchModeCmd = Extract<
EngineCommand['cmd'],
{ DefaultCameraEnableSketchMode: any }
>['DefaultCameraEnableSketchMode'];

and

    const command: Models['ModelingCmdReq_type'] = {/*...*/}
    const cmd:  = command.cmd
    if (typeof cmd !== 'string' && "CameraDragMove" in cmd && this.lossyDataChannel) {
       // do stuff with CameraDragMove cmd
    }

These are good work around but wouldn't want other users of this lib to have to do trick typescript stuff like this.