Introduce ZString ValueStringBuilder

[axunonb]

Significant improvements of performance:

BenchmarkDotNet performance tests for formatters and ISources now show (depending on different input format strings) the following improvements compared to v2.7.0:

• increase in speed by up to 40%
• decrease of GC pressure (collects are only GC Generation 0, allocated memory reduced by up to 60%)

(Formatting measured with a cached parsed Format, and including the result string returned to the caller. Parser was already optimized with PR #187.)

Changes in detail:

• Added Cysharp/ZString as a git subtree to SmartFormat
• Added project SmartFormat.ZString
• Replaced all StringBuilder implementations with Utf16ValueStringBuilder, and all StringOutput implementations with ZStringOutput
• Included SmartFormat.ZString into the nuget package

[codecov]

Codecov Report

Merging #193 (183c326) into version/v3.0 (0d6f514) will decrease coverage by 78%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           version/v3.0    #193      +/-   ##
===============================================
- Coverage            95%     16%     -78%     
===============================================
  Files                50      95      +45     
  Lines              2049   12769   +10720     
===============================================
+ Hits               1939    2082     +143     
- Misses              110   10687   +10577     

Impacted Files	Coverage Δ
...c/SmartFormat.ZString/repo/src/ZString/EnumUtil.cs	0% <ø> (ø)
...rtFormat.ZString/repo/src/ZString/ExceptionUtil.cs	0% <ø> (ø)
...ormat.ZString/repo/src/ZString/FastNumberWriter.cs	0% <ø> (ø)
...artFormat.ZString/repo/src/ZString/FormatHelper.cs	0% <ø> (ø)
...artFormat.ZString/repo/src/ZString/FormatParser.cs	0% <ø> (ø)
...rc/ZString/NestedStringBuilderCreationException.cs	0% <ø> (ø)
...t.ZString/repo/src/ZString/Number/BitOperations.cs	0% <ø> (ø)
...Format.ZString/repo/src/ZString/Number/BufferEx.cs	0% <ø> (ø)
...ormat.ZString/repo/src/ZString/Number/DecimalEx.cs	0% <ø> (ø)
...tFormat.ZString/repo/src/ZString/Number/FloatEx.cs	0% <ø> (ø)
... and 97 more

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