feat(routing): decode pathname early, don't decode params

[ematipico]

Changes

Closes #8516

To partially fix the issue, we had to change some assumptions and expectations about how params are handled.

params aren't decoded anymore, which means that users need to decode the parameters if they need to. While this change might break some users code, it allows us to have better predictability for SSG and SSR.

Changes:

• RenderContext.pathname is decoded early. Usually, browsers and runtimes encode the URLs when creating a Request object, which means that we need to decode them back, so they match the file system paths that we have in RouteData.
• The static build is now able to emit files such as [page]/index.html.
• The function that cleanups the files use paths instead of URL constructs. That was needed because URL does encoding, which is what we want, because now when loading files like [page]/index.html, it will be encoded to something that can't be mapped to the FS file.
• When generating files, we encode the paths.

Testing

Added new tests.

Please let me know if there are more tests I need to add to prevent future regressions.

Docs

withastro/docs#9497

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[github-actions]

This PR is blocked because it contains a major changeset. A reviewer will merge this at the next release if approved.