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/integration/test_01_basic.py

@@ -51,3 +51,50 @@ class TestBasic(PyScriptTest):
         """
         )
         assert self.console.log.lines == [self.PY_COMPLETE, "true false", "<div></div>"]
+
+    def test_paths(self):
+        self.writefile("a.py", "x = 'hello from A'")
+        self.writefile("b.py", "x = 'hello from B'")
+        self.pyscript_run(
+            """
+            <py-config>
+                paths = ["./a.py", "./b.py"]
+            </py-config>
+
+            <py-script>
+                import js
+                import a, b
+                js.console.log(a.x)
+                js.console.log(b.x)
+            </py-script>
+            """
+        )
+        assert self.console.log.lines == [
+            self.PY_COMPLETE,
+            "hello from A",
+            "hello from B",
+        ]
+
+    def test_packages(self):
+        self.pyscript_run(
+            """
+            <py-config>
+                # we use asciitree because it's one of the smallest packages
+                # which are built and distributed with pyodide
+                packages = ["asciitree"]
+            </py-config>
+
+            <py-script>
+                import js
+                import asciitree
+                js.console.log('hello', asciitree.__name__)
+            </py-script>
+            <py-repl></py-repl>
+            """
+        )
+        assert self.console.log.lines == [
+            self.PY_COMPLETE,
+            "Loading asciitree",  # printed by pyodide
+            "Loaded asciitree",  # printed by pyodide
+            "hello asciitree",  # printed by us
+        ]