Refactor how py-script are executed, kill scriptQueue store, introduce pyExec (#881)

Yet another refactoring to untangle the old mess.
Highlights:

base.ts, pyscript.ts and pyrepl.ts were a tangled mess of code, in which each of them interacted with the others in non-obvious ways. Now PyScript is no longer a subclass of BaseEvalElement and it is much simpler. I removed code for handling the attributes std-out and std-err because they are no longer needed with the new display() logic.

The logic for executing python code is now in pyexec.ts: so we are decoupling the process of "finding" the python code (handled by the py-script web component) and the logic to actually execute it. This has many advantages, including the fact that it will be more easily usable by other components (e.g. pyrepl). Also, note that it's called pyexec and not pyeval: in the vast majority of cases in Python you have statements to execute, and almost never expressions to evaluate.

I killed the last remaining global store, scriptQueue tada. As a bonus effect, now we automatically do the correct thing when a <py-script> tag is dynamically added to the DOM (I added a test for it). I did not remove svelte from packages.json, because I don't fully understand the implications: there are various options which mention svelte in rollup.js and tsconfig.json, so it's probably better to kill it in its own PR.

pyexec.ts is also responsible of handling the default target for display() and correct handling/visualization of exceptions. I fixed/improved/added display/output tests in the process.
I also found a problem though, see issue #878, so I improved the test and marked it as xfail.

I removed BaseEvalElement as the superclass of most components. Now the only class which inherits from it is PyRepl. In a follow-up PR, I plan to merge them into a single class and do more cleanup.

During the refactoring, I killed guidGenerator: now instead of generating random py-* IDs which are very hard to read for humans, we generated py-internal-X IDs, where X is 0, 1, 2, 3, etc. This makes writing tests and debugging much easier.

I improved a lot our test machinery: it turns out that PR #829 broke the ability to use/view sourcemaps inside the playwright browser (at least on my machine).
For some reason chromium is unable to find sourcemaps if you use playwrights internal routing. So I reintroduced the http_server fixture which was removed by that PR, and added a pytest option --no-fake-server to use it instead, useful for debugging. By default we are still using the fakeserver though (which is faster and parallelizable).

Similarly, I added --dev which implies --headed and also automatically open chrome dev tools.

pyscriptjs/src/pyexec.ts

@@ -0,0 +1,65 @@
+import { getLogger } from './logger';
+import { ensureUniqueId, addClasses } from './utils';
+import type { Runtime } from './runtime';
+
+const logger = getLogger('pyexec');
+
+export async function pyExec(runtime: Runtime, pysrc: string, outElem: HTMLElement)
+{
+    // this is the python function defined in pyscript.py
+    const set_current_display_target = runtime.globals.get('set_current_display_target');
+    ensureUniqueId(outElem);
+    set_current_display_target(outElem.id);
+    try {
+        try {
+            await runtime.run(pysrc);
+        }
+        catch (err) {
+            // XXX: currently we display exceptions in the same position as
+            // the output. But we probably need a better way to do that,
+            // e.g. allowing plugins to intercept exceptions and display them
+            // in a configurable way.
+            displayPyException(err, outElem);
+        }
+    }
+    finally {
+        set_current_display_target(undefined);
+    }
+}
+
+
+function displayPyException(err: any, errElem: HTMLElement) {
+    //addClasses(errElem, ['py-error'])
+    const pre = document.createElement('pre');
+    pre.className = "py-error";
+
+    if (err.name === "PythonError") {
+        // err.message contains the python-level traceback (i.e. a string
+        // starting with: "Traceback (most recent call last) ..."
+        logger.error("Python exception:\n" + err.message);
+        pre.innerText = err.message;
+    }
+    else {
+        // this is very likely a normal JS exception. The best we can do is to
+        // display it as is.
+        logger.error("Non-python exception:\n" + err);
+        pre.innerText = err;
+    }
+    errElem.appendChild(pre);
+}
+
+
+// XXX this is used by base.ts but should be removed once we complete the refactoring
+export async function pyExecDontHandleErrors(runtime: Runtime, pysrc: string, out: HTMLElement)
+{
+    // this is the python function defined in pyscript.py
+    const set_current_display_target = runtime.globals.get('set_current_display_target');
+    ensureUniqueId(out);
+    set_current_display_target(out.id);
+    try {
+        await runtime.run(pysrc);
+    }
+    finally {
+        set_current_display_target(undefined);
+    }
+}