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/tests/unit/runtime.test.ts

@@ -1,3 +1,4 @@
+import type { AppConfig } from '../../src/pyconfig';
 import { Runtime } from '../../src/runtime';
 import { PyodideRuntime } from '../../src/pyodide';
 
@@ -8,7 +9,8 @@ global.TextDecoder = TextDecoder
 describe('PyodideRuntime', () => {
     let runtime: PyodideRuntime;
     beforeAll(async () => {
-        runtime = new PyodideRuntime();
+        const config: AppConfig = {};
+        runtime = new PyodideRuntime(config);
         /**
          * Since import { loadPyodide } from 'pyodide';
          * is not used inside `src/pyodide.ts`, the function