Refactor suggestions

[karljj1]

Hey. 2 very small refactors I added in our version.

1. Don't recreate the valid chars array each time in Validation.
2. Added TimeSpanFormatOptionsPresets so that internal use values are not exposed in the public enum. This is particularly important to us as we show the contents of the Enum in a dropdown selector for users and having internal use values would cause confusion.

[codecov]

Codecov Report

Merging #290 (82d9d01) into main (0419ced) will increase coverage by 0%.
The diff coverage is 100%.

@@         Coverage Diff         @@
##           main   #290   +/-   ##
===================================
  Coverage    96%    96%           
===================================
  Files        91     91           
  Lines      3180   3178    -2     
===================================
  Hits       3064   3064           
+ Misses      116    114    -2     

Impacted Files	Coverage Δ
...s.Time/Utilities/TimeSpanFormatOptionsConverter.cs	85% <100%> (ø)
...ormat.Extensions.Time/Utilities/TimeSpanUtility.cs	86% <100%> (ø)
src/SmartFormat/Utilities/Validation.cs	100% <100%> (+33%)	⬆️

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[axunonb]

Excellent