Implement ISpanFormattable for DefaultFormatter

[axunonb]

Closes #422

• Implement ISpanFormattable for DefaultFormatter
• Remove usage of stack buffer for ISpanFormattable in ZCharArray
• Make IFormatProvider an optional argument in ZCharArray.Write(...) overloads
• Add unit tests

// Performance test case
Smart.FormatInto(output, null, _placeholder0005Format, 1234567.890123f, 1234567.890123f, 1234567.890123f, 1234567.890123f, 1234567.890123f);

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3737/23H2/2023Update/SunValley3)
13th Gen Intel Core i7-13700K, 1 CPU, 24 logical and 16 physical cores
.NET SDK 8.0.302
  [Host]   : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2
  .NET 8.0 : .NET 8.0.6 (8.0.624.26715), X64 RyuJIT AVX2

Job=.NET 8.0  Runtime=.NET 8.0

| Method                   | N     | Mean          | Error       | StdDev      | Ratio | RatioSD | Gen0       | Allocated    | Alloc Ratio |
|------------------------- |------ |--------------:|------------:|------------:|------:|--------:|-----------:|-------------:|------------:|
| ISpanFormattable         | 100   |     74.502 us |   0.3693 us |   0.3273 us |  8.52 |    0.05 |     4.0283 |      62.5 KB |        2.76 |
| IFormattable             | 100   |     77.927 us |   0.5760 us |   0.5388 us |  8.91 |    0.07 |     5.2490 |     82.03 KB |        3.62 |

ISpanFormattable is 5% faster than IFormattable, with 24% less allocations

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97%. Comparing base (2bf827f) to head (fb41f9f).
Report is 5 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #434   +/-   ##
===================================
  Coverage    97%    97%           
===================================
  Files        96     96           
  Lines      3422   3430    +8     
===================================
+ Hits       3304   3313    +9     
+ Misses      118    117    -1     

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