Add ReadAllParallelAsync (multi-threaded parsing)

[saul]

Thanks @in0finite for the initial implementation. This is largely based off the ideas from #51

Closes #48
Closes #51

[github-actions]

BenchmarkDotNet v0.13.9+228a464e8be6c580ad9408e98f18813f6407fb5a, Ubuntu 22.04.4 LTS (Jammy Jellyfish)
AMD EPYC 7763, 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.401
  [Host]     : .NET 8.0.8 (8.0.824.36612), X64 RyuJIT AVX2
  Job-UFPXIW : .NET 8.0.8 (8.0.824.36612), X64 RyuJIT AVX2
  Job-EDVGXU : .NET 8.0.8 (8.0.824.36612), X64 RyuJIT AVX2

InvocationCount=1  MaxIterationCount=16  UnrollFactor=1  
WarmupCount=1  

Method	Job	Arguments	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
ParseDemo	Job-UFPXIW	/p:Baseline=true	NA	NA	NA	?	?	NA	NA	NA	?
ParseDemo	Job-EDVGXU	Default	2.209 s	0.0179 s	0.0149 s	?	?	6000.0000	2000.0000	579.02 MB	?
ParseDemoParallel	Job-UFPXIW	/p:Baseline=true	NA	NA	NA	?	?	NA	NA	NA	?
ParseDemoParallel	Job-EDVGXU	Default	1.182 s	0.0228 s	0.0190 s	?	?	6000.0000	1000.0000	627.36 MB	?

Benchmarks with issues:
DemoParserBenchmark.ParseDemo: Job-UFPXIW(Arguments=/p:Baseline=true, InvocationCount=1, MaxIterationCount=16, UnrollFactor=1, WarmupCount=1)
DemoParserBenchmark.ParseDemoParallel: Job-UFPXIW(Arguments=/p:Baseline=true, InvocationCount=1, MaxIterationCount=16, UnrollFactor=1, WarmupCount=1)