Add String Variant Validator

[josestg]

This feature basically provides a simple StringValidator for string variants (~string). In the current latest version, if we need a validator for the type VariantExample string, which is a string variant, we have to convert VariantExample to a string in order to use the StringValidator. These changes add a new API that reuses the StringValidator rules for string variants without any conversion.

Before:

type Variant string

const (
   VariantA Variant = "A"
   VariantB Variant = "B"
)

var validator = goval.String().In(string(VariantA), string(VariantB))

After:

type Variant string

const (
   VariantA Variant = "A"
   VariantB Variant = "B"
)

var validator = goval.StringVariant[Variant]().In(VariantA, VariantB)

[codecov-commenter]

Codecov Report

Merging #50 (7edf4e5) into main (db76598) will decrease coverage by 0.27%.
The diff coverage is 93.54%.

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@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
- Coverage   92.07%   91.81%   -0.27%     
==========================================
  Files          13       13              
  Lines         454      464      +10     
==========================================
+ Hits          418      426       +8     
- Misses         27       29       +2     
  Partials        9        9              

Impacted Files	Coverage Δ
errors.go	81.63% <60.00%> (-2.46%)	⬇️
goval.go	97.70% <100.00%> (+0.08%)	⬆️
string.go	100.00% <100.00%> (ø)

[dimasadyaksa]

Hi @josestg , do we need to implement NumberVariant also ?