[Enhancement] mobile and/or responsive screencast dimensions

Context

I want to see the app I'm working on while coding in VS Code.
I think that's the main use case for the screencast feature.

But in practice, it's not usable because of the dimensions of the screencast window.

I suggest:

• We change the default dimensions to match those of a mobile screen and add another button to toggle the current dimensions (for people whose app isn't responsive)
• We add resize capabilities similar to those existing in Edge > device emulation > responsive

We can implement these separately as the first one is straightforward while the second might take some time.

I'm willing to work on the first one If you think it's reasonable.