Add get with default

[kpamnany]

Per this comment.

Our hope, as reflected in one of the added tests, is that this will be non-allocating (unlike the basic get).

[vchuravy]

The ::Union{Nothing, Scope} is likely unecessary (and might even be slower).

[kpamnany]

The ::Union{Nothing, Scope} is how this is called upstream. I've removed it in this PR, but which is right?

[vchuravy]

1. Can you change the title to reflect that this is get with default? Non-allocating is an implementation detail that may or may not be true...
2. Tests are missing, including addition to the benchmarks
3. Likely needs a discussion with JuliaLang/Julia first since we are adding a new API here. E.g. we will want to upstream this eventually.
4. I am not a fan of dropping the default value from the scoped value.

[kpamnany]

1. Can you change the title to reflect that this is get with default? Non-allocating is an implementation detail that may or may not be true...

Done.

2. Tests are missing, including addition to the benchmarks

Added a test. The benchmark does not look current for 1.12 (it uses logstate). It is also using scoped. But most importantly, I need to go read the documentation and understand the framework before I can add to it. I'll try and get to that later this week.

3. Likely needs a discussion with JuliaLang/Julia first since we are adding a new API here. E.g. we will want to upstream this eventually.

Cc: @KristofferC and @topolarity to see if there is anything objectionable to adding this.

4. I am not a fan of dropping the default value from the scoped value.

How would you use the default in the scoped value in this case?

[vchuravy]

How would you use the default in the scoped value in this case?

That should be returned instead of the provided default. The default value is the current value, and get with default should only return the user-provided default if there is no current value.

[NHDaly]

Hey, sorry we left this PR dangling for so long. I understand there was some discussion about whether we should make Base.get and ScopedValues.get the same function, but in discussion today with @JeffBezanson and @kpamnany: We think this PR seems orthogonal to that.

Can we re-review the PR here? It seems like a nice API addition, independent of any of the impacts on the allocation / performance. :)
Are you in favor, @vchuravy?