Integrated core viewer into browser frontend

Temporary solution for accessing the core viewer from the browser frontend. Adds a `view_dump` command, which opens the core viewer in a new tab, and send it the most recent core dump, if there is one.

I still need to do some origin security checks in `postMessage`, and make sure the core viewer can still view files dumped by the node console. Not sure the best way to do this, since the HTTP server serves `build/dev`, apart from the obvious quick hack of copying `core-main.json` from the project root to `build` as part of the Make target.

Once BrowserFS is Doppio-ready, it would be nice to persist the dump and add a browser to the core viewer, so multiple previous files could be chosen from, but that seems to be a while off, and this'll make debugging easier in the meantime.