Request: Enhance "transforms" plugins

`@enhance/ssr` allows for script and css transformations, would it be possible to plug in custom transforms alongside the standard ones provided by `arc-plugin-enhance` here https://github.com/enhance-dev/arc-plugin-enhance/blob/f6dcdd406b60c7b9c8310bb93c6bfb1fff00f20f/src/http/any-catchall/router.mjs#L132-L141

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No idea how this would look for a plugin author...

So far Enhance has intentionally (wisely!) avoided an explicit configuration file. Maybe transforms could be added via a file-based API?
`plugins/transforms/css/index.mjs`
I'm not sold, but it's a thought.

I don't think Enhance _needs_ these sort of plugins, but it would open up some powerful possibilities. On the other hand it could have a negative performance impact and lead to some plugin author/consumer anti-patterns and footguns. On the other _other_ hand, can't save everyone from themselves 😬