+200x Performance increase for huge HTML file | Refactor to TextWriter and more

[r-Larch]

Hi,

This PR brings massive performance at the cost of some breaking changes in the IConverter interface and all derived types.

Key Changes:

• Use TextWriter to build the result Markdown
• Use new performance APIs for text manipulation (net9.0)
• Add polyfills to continue to support legacy target frameworks (net46, netstandard2.0, net6.0, net9.0)
• Cover all target frameworks with tests (net471, net6.0, net9.0) except net46, as Verify.Xunit is not available for it.
• Adding the project ReverseMarkdown.Benchmark to run benchmarks.
• All Tests are green -> this is a refactor without changing behavior.

Note: I suggest dropping support for legacy target frameworks after the net10 release on November 11th.
Dropping legacy support basically means reverting commit b9164ce (makes the code easier to maintain)

Take a look at the benchmark below and note the speed and memory difference.
I tested:

• 1k paragraphs
• 10k paragraphs
• 41k paragraphs (real-world example)

Please feel free to merge or close.
I fully understand if you don't like to introduce the breaking changes.

Kind regards
René

---

Comparing ReverseMarkdown v4.7.1 vs TextWriter approach

Hardware: AMD Ryzen 9 3900X 3.80GHz, 24 cores, 12 physical cores

BenchmarkDotNet v0.15.6, Windows 11 (10.0.26200.6899)
AMD Ryzen 9 3900X 3.80GHz, 1 CPU, 24 logical and 12 physical cores
.NET SDK 9.0.306
  [Host]   : .NET 9.0.10 (9.0.10, 9.0.1025.47515), X64 RyuJIT x86-64-v3
  .NET 9.0 : .NET 9.0.10 (9.0.10, 9.0.1025.47515), X64 RyuJIT x86-64-v3

Job=.NET 9.0 Runtime=.NET 9.0

	Speed	Memory
logarithmic scale on Y
linear scale on Y

Running with Files/1000-paragraphs.html file (size: 442KB) and 1000 paragraphs.

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated	Ratio
ReverseMarkdown TextWriter	25.96 ms	0.519 ms	0.744 ms	3500.0000	1593.7500	1218.7500	24.95 MB	1
ReverseMarkdown v4.7.1	147.7 ms	2.95 ms	8.37 ms	100500.0000	97500.0000	97500.0000	896.35 MB	5.689

Outliers:

• ReverseMarkdown v4.7.1: .NET 9.0 -> 2 outliers were removed (172.08 ms, 174.42 ms)

Running with Files/10k-paragraphs.html file (size: 3.7MB) and 10k paragraphs.

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated	Ratio
ReverseMarkdown TextWriter	0.232 s	0.005 s	0.009 s	20000.0000	5000.0000	1000.0000	210.15 MB	1
ReverseMarkdown v4.7.1	14.08 s	0.280 s	0.747 s	624000.0000	605000.0000	603000.0000	75.27 GB	60.69

Outliers:

• ReverseMarkdown TextWriter: .NET 9.0 -> 1 outlier was removed (265.53 ms)
• ReverseMarkdown v4.7.1: .NET 9.0 -> 2 outliers were removed (17.04 s, 17.23 s)

Running with Files/huge.html file (size: 16MB) and 41312 paragraphs.

Method	Mean	Error	StdDev	Gen0	Gen1	Gen2	Allocated	Ratio
ReverseMarkdown TextWriter	0.944 s	0.0175 s	0.0172 s	86000.0000	20000.0000	3000.0000	955.34 MB	1
ReverseMarkdown v4.7.1	191.611 s	3.7686 s	4.4863 s	2735000.0000	2666000.0000	2659000.0000	640544.94 MB	202.97

[mysticmind]

@r-Larch Appreciate you taking the time to improve performance of the lib by using text writer. But the changes are just too massive that it is going to take a quite a lot of my time for me to check through them. Anyhow, I will see if I can review by next week or so.

[r-Larch]

I see the tests are failing in GitHub actions because of nuget package version outside of dependency constraint in the test project for the targetframework net471.
I saw the warning on my machine but the tests run successfully and were green.
I will fix that soon.

[mysticmind]

@r-Larch I am bit slow on this due to work on other OSS libs (Marten), will definitely get to it this week.