Add support for renderBody mocking

Description

Add a way to mock renderBody when testing components that use <${input.renderBody}/>

Why

There is currently no way to mock or set the renderBody when testing a marko template.

Possible Implementation & Open Questions

From @DylanPiercey:

import { render, createRenderBody } from "@marko/testing-library";

render(template, {
  renderBody: createRenderBody("<h1>Hi!</h1>")
});

Is this something you're interested in working on?

Maybe, depending on guidance from maintainers.

Start at the @marko/testing-library render entry point and inspect how template options are handled; the issue proposes a createRenderBody helper. Confirm the intended API with maintainers, then add coverage showing a template using <${input.renderBody}/> can receive mocked content through render options.