Reduce allocations in EnumsShouldHaveZeroValueAnalyzer

[Alex-Sob]

I would like to suggest PR that is reducing allocations EnumsShouldHaveZeroValueAnalyzer:

• Currently the analyzer allocates ImmutableArray to store zero value fields of an enum type. It can be removed, in the case when there are multiple zero value fields the analyzer just needs to report an error, it can use a flag for that.

• GetZeroValueFields method allocates two enumerators with Where and Cast calls, they can be removed.

• When the analyzer gets zero value field names from analyzer options it allocates string[] with string.Split, ImmutableArray and also a delegate with closure. All those allocations can be removed, particularly string.Split call can be replaced with Span-based splitting.

  There are new MemoryExtensions.Split<T> methods introduced in .NET 9, but they are unavailable for analyzers since they target netstandard2.0. I would probably suggest to provide an internal helper as a simplified version of MemoryExtensions.Split<T> that could be used in analyzers. In this PR, I used it in EnumsShouldHaveZeroValueAnalyzer to reduce allocations.

[Alex-Sob]

@stephentoub There are new MemoryExtensions.Split<T> methods introduced in .NET 9, but they are unavailable for analyzers since they target netstandard2.0. Do you think it's ok to provide a simplified version to be used for analyzers? For example, it could be used in EnumsShouldHaveZeroValueAnalyzer and considerably reduce allocations. By the way, it seems that Range and Index here are local versions of respective BCL types.