Expose seeds in JS SqLambda API

[berekuk]

Discussion in Discord: https://discord.com/channels/1130609991993274510/1223854334761238601

I'll copy my main comment from it here:

Observations:

• Squiggle functions are not exactly pure because they act as generators when they produce randomness; f() = (2 to 3) -> sample; f() == f() is false
• in JS land, f.call() will give you the same number both times
• I don't like this; we should have parity between evaluating Squiggle scripts and calling same Squiggle builtins from JS
• so I guess in JS it should be [result, rngState] = f.call(params, rngState)?
• otherwise things won't be nice in React land, e.g. const value = useMemo(() => f.call(), [f]) won't work
• btw one more complication is that rngState is not a seed; seed -> rngState is kinda slow, and also irreversible
• maybe rngState could be a reducer instance, same as in our common function registry APIs
• but, anyway, right now reducer is not an SqProject, because reducer objects short-lived
• also note that in some cases we'll want to invoke f.call() in multiple code locations, e.g. by passing f to multiple React components; so there should be something for "splitting out RNG into multiple sub-RNGs with unique predictable states" (we'll also need it for parallel execution of Squiggle programs)
• bottom line, we should be explicit about rng state in more places than we expected, and also we'll have to spend more effort on educating users about this if "calling Squiggle from JS" ever becomes popular

[berekuk]

See also: #3168