Refactor py-config and the general initialization logic of the page (#806)

This PR is the first step to improve and rationalize the life-cycle of a pyscript app along the lines of what I described in #763 .
It is not a complete solution, more PRs will follow.
Highlights:

- py-config is no longer a web component: the old code relied on PyConfig.connectedCallback to do some logic, but then if no <py-config> tag was present, we had to introduce a dummy one with the sole goal of activating the callback. Now the logic is much more linear.

- the new pyconfig.ts only contains the code which is needed to parse the config; I also moved some relevant code from utils.ts because it didn't really belong to it

- the old PyConfig class did much more than dealing with the config: in particular, it contained the code to initialize the env and the runtime. Now this logic has been moved directly into main.ts, inside the new PyScriptApp class. I plan to refactor the initialization code in further PRs

- the current code relies too much on global state and global variables, they are everywhere. This PR is a first step to solve the problem by introducing a PyScriptApp class, which will hold all the mutable state of the page. Currently only config is stored there, but eventually I will migrate more state to it, until we will have only one global singleton, globalApp

- thanks to what I described above, I could kill the appConfig svelte store: one less store to kill :).

pyscriptjs/src/pyodide.ts

@@ -1,4 +1,4 @@
-import { Runtime, RuntimeConfig } from './runtime';
+import { Runtime } from './runtime';
 import { getLastPath } from './utils';
 import { getLogger } from './logger';
 import type { PyodideInterface } from 'pyodide';
@@ -16,12 +16,13 @@ export class PyodideRuntime extends Runtime {
     globals: any;
 
     constructor(
+        config,
         src = 'https://cdn.jsdelivr.net/pyodide/v0.21.2/full/pyodide.js',
         name = 'pyodide-default',
         lang = 'python',
     ) {
         logger.info('Runtime config:', { name, lang, src });
-        super();
+        super(config);
         this.src = src;
         this.name = name;
         this.lang = lang;