Refactor pyexec (#1318)

This is some refactoring I did on the way towards resolving pyscript#1313.
I added a new _run_pyscript Python function which executes the code
inside a context manager that sets the display target. We can then
return a JS object wrapper directly from Python.

I moved the "installation" of the pyscript module to loadInterpreter,
and pyimport pyscript_py there and give it a type. This avoids a bunch
of creating and deleting of proxies for pyscript_py and allows us to
give it a type once and for all.

I also did some minor logic cleanup in a few places.

pyscriptjs/tests/unit/setup.ts

@@ -0,0 +1,6 @@
+import { jest } from '@jest/globals';
+import { directoryManifest } from '../../directoryManifest.mjs';
+
+jest.unstable_mockModule('../../src/python_package', async () => ({
+    python_package: await directoryManifest('./src/python/'),
+}));