More plugins: splashscreen and importmap (#938)

This PR move codes from main.ts into two new plugins:

- splashscreen (formerly known as py-loader)
- importmap

The old setting config.autoclose_loader is still supported but deprecated; the new setting is config.splashscreen.autoclose.

Moreover, it does a small refactoring around UserError: now UserErrors are correctly caught even if they are raised from within afterRuntimeLoad.

pyscriptjs/src/plugins/importmap.ts

@@ -0,0 +1,56 @@
+import type { Runtime } from '../runtime';
+import { showWarning } from '../utils';
+import { Plugin } from '../plugin';
+import { getLogger } from '../logger';
+
+const logger = getLogger('plugins/importmap');
+
+type ImportType = { [key: string]: unknown };
+type ImportMapType = {
+    imports: ImportType | null;
+};
+
+export class ImportmapPlugin extends Plugin {
+
+    async afterSetup(runtime: Runtime) {
+        // make importmap ES modules available from python using 'import'.
+        //
+        // XXX: this code can probably be improved because errors are silently
+        // ignored.
+        //
+        // Moreover, it's also wrong because it's async and currently we don't
+        // await the module to be fully registered before executing the code
+        // inside py-script. It's also unclear whether we want to wait or not
+        // (or maybe only wait only if we do an actual 'import'?)
+        for (const node of document.querySelectorAll("script[type='importmap']")) {
+            const importmap: ImportMapType = (() => {
+                try {
+                    return JSON.parse(node.textContent) as ImportMapType;
+                }
+                catch(error) {
+                    showWarning("Failed to parse import map: " + error.message);
+                }
+            })();
+
+            if (importmap?.imports == null) continue;
+
+            for (const [name, url] of Object.entries(importmap.imports)) {
+                if (typeof name != 'string' || typeof url != 'string') continue;
+
+                let exports: object;
+                try {
+                    // XXX: pyodide doesn't like Module(), failing with
+                    // "can't read 'name' of undefined" at import time
+                    exports = { ...(await import(url)) } as object;
+                } catch {
+                    logger.warn(`failed to fetch '${url}' for '${name}'`);
+                    continue;
+                }
+
+                logger.info("Registering JS module", name);
+                runtime.registerJsModule(name, exports);
+            }
+        }
+    }
+
+}