Introduce a Plugin system, implement <py-terminal> as a plugin (#917)

This PR does two things:

1. introduce the first minimal version of a Plugin system. Plugins are subclasses of the Plugin class, and pyscript will call the relevant methods/hooks at the right time. Currently, I have added only the minimal sets of hooks which were needed for this PR.

2. Implement <py-terminal> as a plugin.

pyscriptjs/src/plugin.ts

@@ -0,0 +1,70 @@
+import type { PyScriptApp } from './main';
+import type { AppConfig } from './pyconfig';
+import type { Runtime } from './runtime';
+
+export class Plugin {
+
+    /** Validate the configuration of the plugin and handle default values.
+     *
+     * Individual plugins are expected to check that the config keys/sections
+     * which are relevant to them contains valid values, and to raise an error
+     * if they contains unknown keys.
+     *
+     * This is also a good place where set default values for those keys which
+     * are not specified by the user.
+     *
+     * This hook should **NOT** contain expensive operations, else it delays
+     * the download of the python interpreter which is initiated later.
+     */
+    configure(config: AppConfig) {
+    }
+
+    /** The preliminary initialization phase is complete and we are about to
+     * download and launch the Python interpreter.
+     *
+     * We can assume that the page is already shown to the user and that the
+     * DOM content has been loaded. This is a good place where to add tags to
+     * the DOM, if needed.
+     *
+     * This hook should **NOT** contain expensive operations, else it delays
+     * the download of the python interpreter which is initiated later.
+     */
+    beforeLaunch(config: AppConfig) {
+    }
+
+    /** The Python interpreter has been launched, the virtualenv has been
+      * installed and we are ready to execute user code.
+      *
+      * The <py-script> tags will be executed after this hook.
+      */
+    afterSetup(runtime: Runtime) {
+    }
+}
+
+
+export class PluginManager {
+    _plugins: Plugin[];
+
+    constructor() {
+        this._plugins = [];
+    }
+
+    add(p: Plugin) {
+        this._plugins.push(p);
+    }
+
+    configure(config: AppConfig) {
+        for (const p of this._plugins)
+            p.configure(config);
+    }
+
+    beforeLaunch(config: AppConfig) {
+        for (const p of this._plugins)
+            p.beforeLaunch(config);
+    }
+
+    afterSetup(runtime: Runtime) {
+        for (const p of this._plugins)
+            p.afterSetup(runtime);
+    }
+}