New hook onRenderHtmlEnd()

[brillout]

Description

Called after the HTML is fully generated (after Vike injects assets).

Use case: HTML transformers, such as critical CSS inlining #1975.

Workaround in the meantime:

• SSG: crawl HTML files in dist/client/ and apply transformer against each HTML file.
• SSR: run transformer after renderPage().

(Note to maintainers: ideally the name should be swapped with the already existing onAfterRenderHtml().)