Parse the root tokens in the rsx!{} macro

This parsing path emerges directly from the macro call, with RsxRender being the primary entrance into parsing. This feature must support:

• Optionally rendering if the in XYZ pattern is present
• Fragments as top-level element (through ambiguous)
• Components as top-level element (through ambiguous)
• Tags as top-level elements (through ambiguous)
• Good errors if parsing fails

Any errors in using rsx! will likely occur when people start using it, so the first errors must be really helpful.