[playground] Allow resizing of playground sections

[aralroca]

Related to #158

In the Brisa Playground code size, preview, and terminal are modifiable by developers.

[chankruze]

@enzonotario @aralroca should we use any external package or should we try with vanilla css ?

As we already have automatic layout set to true, here
https://github.com/chankruze/brisa/blob/a97caf0b0c7921308ac525282ff5d101620ed442/packages/www/src/pages/playground/index.tsx#L71

const editor = monaco.editor.create(document.querySelector('#code-editor'), {
    theme: document.body.classList.contains('dark') ? "vs-dark" : "vs-light",
        automaticLayout: true,
    });

[enzonotario]

I was trying to use an existing WebComponent (https://opensource.adobe.com/spectrum-web-components/components/split-view/), with no success so far.

I think it would be better to use an existing solution, because do it from zero, considering Responsive, Accessibility, etc, would be long.

Feel free to propose a solution. If not, I'll give it a try on weekend.