<link rel=preload> interpreters

[dfabulich]

Apropos #178, we should be able to add a <link rel=preload> to the HTML of https://iplayif.com/?story=https%3A%2F%2Fifarchive.org%2Fif-archive%2Fgames%2Fzcode%2FLostPig.z8 to begin downloading the interpreter script (bocfel.js, glulxe.js, etc.) and its corresponding WASM file (<link rel=preload as=fetch>) in the built-in HTML.

front-page.ts could look up the file out of a cache (especially for IF Archive links), detect the type, and add the preloads to the HTML.