feat: add u.RandomLetters helper

[moul]

No description provided.

[github-actions]

Expected release notes (by @moul)

features:
add u.RandomLetters helper (f979538)

• I hereby acknowledge these release notes are 🥙 AWESOME 🥙

[codecov]

Codecov Report

Merging #45 (f979538) into master (f34f003) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   74.32%   74.58%   +0.25%     
==========================================
  Files          12       13       +1     
  Lines         296      299       +3     
==========================================
+ Hits          220      223       +3     
  Misses         47       47              
  Partials       29       29              

Flag	Coverage Δ
unittests	74.58% <100.00%> (+0.25%)	⬆️

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

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[github-actions]

🎉 This PR is included in version 1.22.0 🎉

The release is available on GitHub release

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