feat: add (optional) defaultValue parameters to firstValueFrom and lastValueFrom

[cartant]

Description:

This PR adds an optional defaultValue parameter to firstValueFrom and lastValueFrom. The parameters are added using an overload to avoid this issue (for some usage, at least).

The first and last operators both support defaults and I see no reason why firstValueFrom and lastValueFrom should not do the same. Sure, you could apply defaultIfEmpty to the source, but you say the same for first and last. And, IMO, including defaultValue in the signatures serves as a reminder that the functions expect a value and that not having an available value is an unexpected/error situation.

Also, supporting default values would make implementing a user-land equivalent that resolves with an Option straightforward. (IMO, introducing a Option isn't something we should do while there are bunch of APIs that throw for empty sources.)

Related issue (if exists): Nope

[benlesh]

@cartant I think this should probably be in a config/options object. I say this because there's a strong possibility that this will support AbortSignal at some point:

const ac = new AbortController();

firstValueFrom(source$, { defaultValue: 2, signal: ac.signal });

[cartant]

@benlesh

I think this should probably be in a config/options object.

Nice catch. Done.