Add `webc:bundler` abstraction for user to handle bundling themselves using their own bundler

Tried to make a customTransform called esbuild to transform esm javascript that imports node modules and local modules

src
- page.webc
- live-reload.js
- diagrams.js

src/page.webc

<script webc:type="esbuild" src="./live-reload.js"></script>

src/diagrams.js

import mermaid from 'mermaid' // or any other installed node_module
/* magic */
export default {}

src/live-reload.js

import diagrams from './diagrams.js'

/* code that uses diagrams */

Looks like a common enough use case. I don't think customTransforms were made for this. And transforms are not sufficient for this since get the content and not the src paths. And they are executed once per node, instead of once per compilation

Feels like we need a similar abstraction for bundled assets which operate on src paths

<script webc:bundler="esbuild" src="./live-reload.js"></script>

Thoughts?

Start by reviewing the existing customTransform behavior and the compilation lifecycle, focusing on how transforms receive content and are invoked per node. Compare that with the proposed webc:bundler attribute and its src-path-based, per-compilation behavior; done means the abstraction and its expected user-facing behavior are defined and supported.