feat(rollup-plugin-html): suport transform on input html

[bashmish]

What I did

1. Add 2nd param to "plugin.api.addHtmlTransformer" to specify the stage of the transform - "input" or "output"
2. By default keep it "output" as before, for backwards compatibility
3. When "input" is set, it will apply to the input HTML instead of output HTML, allowing to add assets and stuff like that that later will go through the pipeline

GOOD TO MENTION: Do not confuse with Vite's "order", their transforms always apply to the HTML before it goes through the pipeline, and the "order" only defines the priority of transform functions. If we need to add smth like this in the future, we gonna use a 3rd param, e.g. "priority", for that, but this PR has nothing to do with that, just my 2 cents.

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Latest commit: 44fff34

The changes in this PR will be included in the next version bump.

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Name	Type
@web/rollup-plugin-html	Minor

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[thepassle]

should we add some error handling here?

[bashmish]

good one, in general the exception will be associated with a particular plugin, so locating the problem should be easy enough, but it can be improved always but making it more specific, just maybe logical not to mix up here with this change since the behavior mimics that of the output html transform
or do you want to do it in this PR?

[thepassle]

I think even just a nice readable error message "transform hook threw an error" or something would be fine, can indeed improve in the future.

(not for this PR, but) Typically when I implement plugin systems in my tooling I require them to have a name, e.g.:

const fooPlugin = {
  name: 'foo-plugin',
  someHook: () => {}
}

That way you can throw really helpful error messages that include the name of the plugin it threw on