Add Benchmark and minor performance refactor

[jafin]

Benchmarking Project Setup:

• Added a new benchmarking project ClosedXML.Report.Benchmarks to measure report generation performance using the BenchmarkDotNet library. (ClosedXML.Report.Benchmarks/Benchmarks/ReportBenchmarks.cs, ClosedXML.Report.Benchmarks/Benchmarks/ReportBenchmarks.csR1-R46)

Refactoring:

• Refactored TempSheetBuffer to replace individual row and column variables with a CellPosition struct, simplifying cell position management and improving code readability. (ClosedXML.Report/Excel/TempSheetBuffer.cs, ClosedXML.Report/Excel/TempSheetBuffer.csL13-R135)

• Updated SubtotalSummaryFunc to use modern C# features such as target-typed new() and pattern matching, and replaced verbose property definitions with expression-bodied members. (ClosedXML.Report/Excel/SubtotalSummaryFunc.cs, [1] [2] [3]

• Track LastCellUsed for small perf gain.

Motivations

We have code that generates spreadsheets, some are taking an increasing amount of time and memory to create. A lot of the performance is in the underlying ClosedXml, not so much reporting lib, but these changes add a slight drop in raw and even slighter performance boost in general.

Benchmark this PR

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
ReportGeneration	2.867 s	0.0552 s	0.0737 s	131000	45000	5000	2.04 GB
ReportGeneration	2.753 s	0.0534 s	0.0500 s	131000	45000	5000	2.04 GB

Benchmark Existing code

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated
ReportGeneration	2.940 s	0.0545 s	0.0968 s	134000	45000	5000	2.08 GB
ReportGeneration	2.902 s	0.0570 s	0.0679 s	134000	44000	5000	2.08 GB

Significant amount of ram allocation still! ⚠️

Flamegraph shows a lot of time is spent in the underlying ClosedXML lib, one hot path for us is in conditional formatting application. I'll see if I can review that in the ClosedXML project.

Test changes

I amended one expected test, it was failing I Think the change is justified.
Here is the existing test expectation

Here is the amended: note the formatting on the Item 6 cell. All the other cells follow this pattern, so I think it made sense that this cell also should be rendered the same.

I actually think all the cells should have the border formatting, but that is for another day.