Retool esbuild configuration and add "Islands Architecture"

[jaredcwhite]

This builds upon the Declarative Shadow DOM support in #763 and enables a real (and fullstack!) Islands Architecture paradigm for building Bridgetown websites. It's a fairly straightforward solution to a problem I've been noodling on for quite some time now. This is gonna be big.

TL;DR: (I still have to write up the long docs, haha)

Bridgetown + esbuild now supports a src/_islands folder. Inside this folder, any top-level .js/.js.rb file automatically becomes an entrypoint. In addition, you can create folders for each island which could contain Bridgetown components with sidecar JS/CSS files (all supporting DSD of course).

With the island entrypoint(s) built, we now turn things over to Zach Leatherman's excellent <is-land> component. This enables lazy-loaded UI by default. For example, this island only loads the island JS upon click:

<is-land on:interaction import="<%= asset_path 'islands/breezy.js' %>">
  <%= render Breezy::MyComponent.new %>
</is-land>

With DSD, you can server-render the island's initial state including styling, and then upon click just load the frontend and hydrate accordingly. Probably most islands though will use on:visible so the JS can load as soon as the island is on screen.

Note
The <is-land> component automatically polyfills DSD, as does my Turbo + Turbo Shadow combo, so either way we're covered.

But wait, there's more! The bridgetown-routes plugin has been updated to support Roda code/templates inside of src/_islands/routes. This means you can create a full vertical slice of your site/application. For example, you could create a "purchasing island" which includes backend routes for adding to cart or checking out, new dynamic pages to facilitate the checkout, and the static pages with dynamically-updating product purchasing details (price, Add to Cart button, etc.). In addition, because these route URLs are all hanging off of /islands/, it'll make punching a hole through on Render a piece of cake in static + SSR hybrid architectures.

What does this mean?

I see this as essentially becoming The Way:tm: to build Bridgetown applications going forward. Future efforts around handling frontend assets, feature-specific UI components, styling, layouts, Roda backend, etc. can all be funneled through this islands paradigm. It's a path to fullstack modularity, but with enough of the appeal of micro-services and micro-frontends to make scaling larger and more ambitious projects straightforward.

I'm very excited to see how we can build upon and improve this idea!

---

Other code in this PR:

• Remove previously deprecated non-Zeitwerk plugin support
• Fix some subtle Watcher bugs
• Improve esbuild glob functionality