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/utils.ts

@@ -46,10 +46,8 @@ export function showWarning(msg: string, messageType: "text" | "html" = "text"):
 }
 
 export function handleFetchError(e: Error, singleFile: string) {
-    //Should we still export full error contents to console?
-    // XXX: What happens if I make a typo? i.e. a web server is being used but a file
-    // that doesn't exist is being accessed. We should cover this case as well.
-    console.warn(`Caught an error in fetchPaths:\r\n ${e.toString()}`);
+    // XXX: inspecting the error message to understand what happened is very
+    // fragile. We need a better solution.
     let errorContent: string;
     if (e.message.includes('Failed to fetch')) {
         errorContent = `<p>PyScript: Access to local files
@@ -66,11 +64,7 @@ export function handleFetchError(e: Error, singleFile: string) {
     } else {
         errorContent = `<p>PyScript encountered an error while loading from file: ${e.message} </p>`;
     }
-    // We need to create the banner because `handleFetchError` is called before we
-    // use withUserErrorHandler in main.js we are also disabling the log message
-    // because it will be logged by the uncaught exception in promise.
-    _createAlertBanner(errorContent, "error", "html", false);
-    throw new UserError(errorContent);
+    throw new UserError(errorContent, "html");
 }
 
 export function readTextFromPath(path: string) {