Add js.Batch

[bep]

js.Batch (new template func) takes an ID and a store (e.g. site.Store) and returns aBatcher instance:

type Batcher interface {
	UseScript(id string) BatcherScript
	UseScriptMany(id string) BatcherScriptMany
	Build() (*Package, error)
}

Benefits

• Code splitting
• UseScriptMany allows creation of many JS modules and instances with coordinated instantiation in a user defined callback function. Typical use case is React/JSX components where the user provided callback function could be set up as:

import * as ReactDOM from 'react-dom/client';
import * as React from 'react';

export default function Callback(modules) {
	for (const module of modules) {
		for (const instance of module.instances) {
			/* This is a convention in this project. */
			let elId = `${module.id}-${instance.id}`;
			let el = document.getElementById(elId);
			if (!el) {
				console.warn(`Element with id ${elId} not found`);
				continue;
			}
			const root = ReactDOM.createRoot(el);
			const reactEl = React.createElement(module.mod, instance.params);
			root.render(reactEl);
		}
	}
}

Limitations

• There is a import order issue with ESBuild's current implementation, see this issue

Fixes #12626