.NET 8.0 and a few other perf improvements

[MihaZupan]

• Added a .NET 8.0 TFM
• Polyfills for a few new functionalities (Ascii, SearchValues, ContainsAnyExcept ...)
• Replaced the CharacterMap.IndexOfOpeningCharacter implementation with a call to IndexOfAny(SearchValues)
  • Don't need manual vectorization code in Markdig for this anymore
  • Also get AVX2 paths and ARM64 acceleration "for free"
• Changed CodeInlineParser.Match to make use of vectorized helpers
• Sped up WriteEscape & WriteEscapeUrl rendering (vectorized scanning via SearchValues)
• A few random alloc / codegen improvements

A few rough numbers from parsing a large markdown document with precise source locations (.NET 8 perf improvements blogpost).
Both main and pr numbers are from 8.0

Method	Job	Mean	Error	Ratio
Parse	main	2,762.2 us	13.73 us	1.00
Parse	pr	2,509.3 us	13.97 us	0.91
ParseAdvanced	main	7,991.8 us	49.89 us	1.00
ParseAdvanced	pr	7,758.2 us	48.96 us	0.97
Render	main	954.6 us	3.39 us	1.00
Render	pr	710.6 us	2.42 us	0.74
RenderAdvanced	main	1,301.3 us	4.76 us	1.00
RenderAdvanced	pr	1,068.6 us	20.04 us	0.82

Including 7.0 vs 8.0 comparison

Method	Job	Runtime	Mean	Error	Ratio
Parse	main	.NET 7.0	3,125.7 us	18.30 us	1.00
Parse	pr	.NET 8.0	2,486.9 us	9.08 us	0.80
ParseAdvanced	main	.NET 7.0	8,481.4 us	35.35 us	1.00
ParseAdvanced	pr	.NET 8.0	7,682.9 us	17.01 us	0.91
Render	main	.NET 7.0	1,140.1 us	4.04 us	1.00
Render	pr	.NET 8.0	711.8 us	1.49 us	0.62
RenderAdvanced	main	.NET 7.0	1,548.4 us	5.30 us	1.00
RenderAdvanced	pr	.NET 8.0	1,047.4 us	4.44 us	0.68

[xoofx]

Nice upgrade! Thanks @MihaZupan!