fix: RTS peer dep w/ Vite

[rschristian]

Fixes: preactjs/preset-vite#117

Vite's pretty weird here; when RTS isn't installed...

• and the default export is used, it'll have no problem
• and a wildcard import is used, it'll emit a warning
• and a named import is used, it'll fall right over and fail to build (as the linked issue shows)

Alternatively, maybe it's worth reverting making RTS an optional peer dep altogether as there's also this funky issue

Edit: Worth looking at in the next major, maybe keep /prerender as the only subpath export. I don't want to break anything now though.