Detect static routes and avoid re-render

[Rich-Harris]

In a lot of cases (e.g. this about page), there's no need to hydrate the page because nothing is interactive. We can just wait for the first navigation event.

This is perhaps something that belongs in Component.render(...):

const { html, head, css, interactive } = Component.render(data);

// if `interactive === false`, set `window.SAPPER_IS_INTERACTIVE = false`,
// then we don't bother with the initial `navigate(new URL(window.location))`