fix: memory leak in terminal process (partially)

[SimonSiefke]

Related to #276958. Registering three disposables, similar to how other disposables are registered.

Results

Results still show the same promise memory leak as before. So it seems like there could still be other leaks somewhere related to this promise:

this._processStartupComplete = new Promise<void>(c => {
  this.onProcessReady(() => c());
});