feat(standalone): Use data attribute for 404 tracking (avoid separate script)

[cziegeler]

Currently we create two scripts standalone and standalone-404 - which are mainly identical, with the exception that the latter is used for tracking 404 responses. With the recent introduction of using the navigation API into the standalone script, it makes sense to merge the functionality into a single script.

This patch uses the data-status attribute to inform the script of a 404 response, instead of using a separate script.

For now we should still create the standalone-404 until we can phase it out

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Commits

* [a793f13](https://github.com/adobe/helix-rum-js/commit/a793f13851080ce85bcfaf80e03f9a1da323c6b5) :white_check_mark: (latest)

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:tada: This PR is included in version 2.10.0 :tada:

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