feat: Add possibility to register custom formats and their validators.

[AlexMinaev19]

Resolves #152

Added new configuration option Formats that allows you to register custom formats and their validators.

Usage:

v, err := NewValidator(
    document,
    config.WithFormatAssertions(), // required to enable format assertions
    config.WithCustomFormat("capital", func(v any) error {
        s, ok := v.(string)
	if !ok {
		return fmt.Errorf("expected string")
	}

	if s == "" {
		return nil
	}

	r := []rune(s)[0]

	if !unicode.IsUpper(r) {
		return fmt.Errorf("expected first latter to be uppercase")
	}

	return nil
    }),
)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.89%. Comparing base (8660f37) to head (8dd6f32).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #166      +/-   ##
==========================================
+ Coverage   96.88%   96.89%   +0.01%     
==========================================
  Files          34       34              
  Lines        3791     3804      +13     
==========================================
+ Hits         3673     3686      +13     
  Misses        118      118              

Flag	Coverage Δ
unittests	96.89% <100.00%> (+0.01%)	⬆️

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