Rewrite as go-randomstring (v1.0.0)

[gbbocchini]

What changed

Full rewrite of the package, renamed from go-string-randomizer to go-randomstring
(module github.com/gbbocchini/go-randomstring, package randomstring).

Fixes

• Character-set bug — three collections contained "ABCDEUFGHIJKLMNOPQRSTUVWXYZ"
  (missing F, duplicated U). Sets are now composed from LowerLetters/UpperLetters/Digits,
  so that class of typo can't recur.
• Deprecated/unsafe RNG — replaced rand.Seed on the global rand with math/rand/v2 local
  sources; no per-call reseeding, goroutine-safe.

New API

r := randomstring.Randomizer{
    Universe: randomstring.LowerUpperDigits,
    Length:   13,
    Unique:   true,  // distinct within a batch
    Secure:   true,  // crypto/rand for passwords/tokens
    Seed:     1,     // deterministic output
}
id, err := r.GenerateOne()
ids, err := r.Generate(10_000)

• Error-last returns: ([]string, error).
• UniquePermutations() *big.Int (exact — replaces the float64 version).
• Crypto-secure mode via the Secure field.
• ASCII fast path for speed; Unicode universes supported.

Docs & tests

• pkg.go.dev-ready package doc (doc.go) and doc comments on every exported symbol.
• Runnable examples (example_test.go) + unit tests + a benchmark.
• Rewritten README with badges, install, and usage guide.

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