`LayoutBuilder\Output::render()` echos content, but `[layout]` shortcode should return, not echo

[agopshi]

Right now, the [layout] shortcode essentially echos out the layout because LayoutBuilder\Output::render() echos it. This isn't a big problem on pages that only use a layout and nothing else, but it causes problems for pages that mix one or more layouts and other content.

Proposed fix:

1. Add an $echo = true parameter to LayoutBuilder\Output::render(). This will maintain backwards compatibility, and if it's set to false, it should return the output.
2. Change the [layout] shortcode to use $echo = false when calling LayoutBuilder\Output::render(). This should fix the shortcode without breaking any existing code.

[ghost]

Added $return = false parameter instead of $echo = true to keep return/direct output parameter name consistency.