show compiled output size

[Rich-Harris]

This is mostly a placeholder so that we can close sveltejs/svelte-repl#85.

It raises a lot of tricky design questions — would we need to bundle using the specified compiler options? if so is that the bundle that we run (e.g. if using ssr mode is the app non-interactive?) should we exclude external dependencies? where does the output go?

We should figure out answers to all these questions before doing any implementation work.

[pngwn]

I actually did this a while ago in my own REPL and one of the problems is just how damn expensive minification is. It ended up stacking up work in the worker. Even though this didn't impact the normal functioning in the real, with a lot of edits it would leave a significant delay on the bundle size updating. In the end I did some aggressive throttling/ debouncing which kinda solved it but I didn't like it.