Show current pathname

[Rich-Harris]

When we add SvelteKit material, it'll become necessary to show the current pathname somehow. (I'm actually not sure if it's possible to track this from outside the iframe, but I'll be optimistic and assume that it is.)

We probably don't want this for the Svelte material.

[Rich-Harris]

Solved by injecting a client that posts a message. It won't work if config.kit.files.template is something other than src/app.html, but we can cross that bridge another time.

Currently the URL bar is always visible, but it'd be easy to hide for Svelte sections if we want to