feat(plugin): create plugin to integrate modules

[dsurducan]

Allows clients to create "modules". See my test modules here: https://github.com/asanehisa/widgets-test-site/tree/main/src/modules
Hosted here: https://www.yext.com/s/4059983/yextsites/134396/branch/96516/deploys/recent?b=4059983

example use case: to allow clients to put react Search SDK on their non react pages (similar to how we used to iframe the Search Theme in the past)

All a user would need to do to have this work is:

1. create a separate folder under the "modules" subfolder in the pages-starter for each module with the name of the folder corresponding to the name of the module (ex. "search") which will be what is used when packaging into a umd module (ex. search.umd.js)

In each folder they can have:

• The main module file (ex search.tsx) (this will contain all of the logic and configuration that pages allows you to provide, some variable of type Module is required. If ModuleConfig is configured, it'll use the provided name.)
• tailwind.config.ts file
• postcss.config.cjs file that specifies the path of the tailwind config file they want the module to use
  module.exports = { plugins: { "tailwindcss": {config: './tailwind-config.ts' }, "autoprefixer": {}, }, };
  Also ensure the css code (if used) is scoped properly to the widget. It can affect code in the non-react site if not.

This also adds response headers to the config.yaml such that different sites can import the widget. This can be manually changed to only allow certain sites to use the widget.

Here is a test site to see how modules appear.
https://vibrantly-plain-lark.pgsdemo.com/test.html

To inject a module into non-react site the following is required:

<div id="moduleName"> </div>
<script type="module" src="moduleUrl"></script>

Here's my most recent instructions / setup guide:
https://snapdragon-hat-825.notion.site/Widgets-in-PagesJS-fc01b4e61005482e92ec4ebee462e603?pvs=4