Alternative name: Make %Intl.Segmenter%.[[AvailableLocales]] be the full set of all syntactically valid locales (with some caveats like canonicalization/extensions)

Rationale

While most Intl services do require passing a locale for a correct behaviour at runtime, the Segmenter service is in this weird position where it supports almost all locales you throw at it, and the provided locale is just used as a suggestion to segment certain special cases.

This apparently makes it difficult for libraries such as ICU4X to determine if a locale is on their list of [[AvailableLocales]] or not; in that case, only a couple of locales ("km", "lo", "my", "th") are "supported" in the sense that they load some amount of data for them on their data provider. However, the rest of locales are very much "supported", they just don't load locale specific data at runtime. (asking for @sffc's help to add more context about this)

What then? Well, if virtually all locales are "supported" by Segmenter, why not just consider all (see alternative name) syntactically valid locales as supported locales for that service? This would mean making APIs such as Intl.Segmenter.supportedLocalesOf always return everything, which doesn't sound too bad for a service that is basically a low level text processing utility.