Treat earliest possible Baseline date differently

There's a number of Baseline banners where the "been available across browsers" date we show is just the release date of Edge's first version, as that's the "newest" browser in the Baseline browser set, but these features have been available for much longer: such as the Array() constructor.

There's some discussion of this upstream, and we can automatically compute this "earliest possible" date.

Perhaps we should show something a little different in the banner if the date matches the earliest possible date, the current string is:

It’s been available across browsers since July 2015.

It could be:

It’s been available across browsers since July 2015 or earlier.

Or even (with the number of years computed):

It’s been available across browsers for at least 10 years.

I think the last stresses both the length of time it's been available, and the fuzziness of the date best to me.

In the future we could even reuse the logic to use a smaller inline variant of the banner, or something like that.

@mdn/content-team thoughts?

Start by locating the Baseline banner implementation and reviewing the upstream web-features discussion linked in the issue. Decide how banners should distinguish an earliest-possible date, then verify the chosen wording and computed duration in the relevant frontend tests.