add ITerminalContribution.layout

[meganrogge]

          It's a bit ugly to expose this, how about instead we add `ITerminalContribution.layout` and call it for all contributions within `TerminalInstance`? That seems like it would be useful for future contributions.

For this case we would only need to listen to the first layout:

layout() {
  if (!this._doneLayout) {
    this._accessibleBufferWidget = this._instantiationService.createInstance(AccessibleBufferWidget, this._instance.instanceId, xterm);
    this._doneLayout = true;
  }
}

Originally posted by @Tyriar in #176413 (comment)