feat(string): impl variant validator and ensure backward compatibility

[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 #49 (2fabefb) into develop (78cf99f) will increase coverage by 0.03%.
The diff coverage is 100.00%.

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@@             Coverage Diff             @@
##           develop      #49      +/-   ##
===========================================
+ Coverage    91.77%   91.81%   +0.03%     
===========================================
  Files           13       13              
  Lines          462      464       +2     
===========================================
+ Hits           424      426       +2     
  Misses          29       29              
  Partials         9        9              

Impacted Files	Coverage Δ
string.go	100.00% <100.00%> (ø)