feat(oversight): add trend indicators when looking at date-range chart

[trieloff]

See https://oversight-trends--helix-website--adobe.aem.live/tools/oversight/explorer.html?domain=www.aem.live&filter=&view=month&checkpoint=cwv-lcp&conversion.checkpoint=click

Note the little line below some of the key metrics, indicating if the trend shows an improvement over time

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Commits

* [632a180](https://github.com/adobe/helix-website/commit/632a180c26a4091b27cacb9c9b1890e69d48f1eb) :white_check_mark: (latest)