Just bundle embed.js via webpack with target:umd and the name "embed" (or pick one)

demo.html

<script src="/path/to/embed.umd.js"></script>

<div id="foo"></div>
<script>
var parent = document.getElementById('foo')  
embed.renderWidget(
  'foo',       // widget name
  { a: 'b' },  // props
  parent       // render into this
);
</script>

embed.js

import { h, render } from 'preact';
// could use webpack require context here, but we'll be verbose:
import Foo from './components/foo';
import Bar from 'component-from-some-other-repo';

const COMPONENTS = {
  Foo,
  Bar
};

// case-insensitive lookup of a named component ("widget")
export function getWidget(name) {
  for (let i in COMPONENTS) {
    if (COMPONENTS.hasOwnProperty(i) && i.toLowerCase()===name.toLowerCase()) {
      return COMPONENTS[i];
    }
  }
}

export function renderWidget(name, props, parent) {
  let Widget = getWidget(name);
  render(<Widget {...props} />, parent, parent.firstChild);
}