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/components/pyloader.ts

@@ -1,42 +0,0 @@
-import { getLogger } from '../logger';
-
-const logger = getLogger('py-loader');
-
-export class PyLoader extends HTMLElement {
-    widths: string[];
-    label: string;
-    mount_name: string;
-    details: HTMLElement;
-    operation: HTMLElement;
-    constructor() {
-        super();
-    }
-
-    connectedCallback() {
-        this.innerHTML = `<div id="pyscript_loading_splash" class="py-overlay">
-        <div class="py-pop-up">
-        <div class="smooth spinner"></div>
-        <div id="pyscript-loading-label" class="label">
-          <div id="pyscript-operation-details">
-          </div>
-        </div>
-        </div>
-      </div>`;
-        this.mount_name = this.id.split('-').join('_');
-        this.operation = document.getElementById('pyscript-operation');
-        this.details = document.getElementById('pyscript-operation-details');
-    }
-
-    log(msg: string) {
-        // loader messages are showed both in the HTML and in the console
-        logger.info(msg);
-        const newLog = document.createElement('p');
-        newLog.innerText = msg;
-        this.details.appendChild(newLog);
-    }
-
-    close() {
-        logger.info('Closing');
-        this.remove();
-    }
-}