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/pyterminal.ts

@@ -1,5 +1,6 @@
 import type { PyScriptApp } from '../main';
 import type { AppConfig } from '../pyconfig';
+import type { Runtime } from '../runtime';
 import { Plugin } from '../plugin';
 import { UserError } from "../exceptions"
 import { getLogger } from '../logger';
@@ -46,7 +47,7 @@ export class PyTerminalPlugin extends Plugin {
         }
     }
 
-    afterSetup() {
+    afterSetup(runtime: Runtime) {
         // the Python interpreter has been initialized and we are ready to
         // execute user code:
         //