Document the dev environment setup

[Windvis]

I wanted to give #43 a go, but I can't figure out how to setup the dev environment.

I cloned both this and the svelte repo. Installed dependencies in the site folder, ran the needed commands there, and npm linked the repl repo into the site repo. After that I get compile errors when building the website though, so I'm probably missing something.

✗ client
../../svelte-repl/src/SplitPane.svelte (21:1)
'transition_in' is not exported by ../../svelte-repl/node_modules/svelte/internal.mjs
19:     const { top, bottom, left, right } = refs.container.getBoundingClientRect();
20: 
21:     const extents = type === 'vertical' ? [top, bottom] : [left, right];
      ^
22: 
23:     const px = yootils.clamp(
✗ server
../../svelte-repl/src/SplitPane.svelte (3:1)
'add_attribute' is not exported by ../../svelte-repl/node_modules/svelte/internal.mjs
1: <script>
2:   import * as yootils from 'yootils';
3:   import { createEventDispatcher } from 'svelte';
     ^
4: 
5:   const dispatch = createEventDispatcher();

Any pointers?

[Wolfr]

This would be helpful as I also wanted to fix an issue with the REPL and this kind of discourages me to go and figure it out.

Hint: just running npm run dev is not enough.