fix: length validation for utf8 strings

[maximdanilchenko]

Hi! Current string length validation made with go's standard len function works incorrectly with non ascii characters. So if there any non-english symbols "maxLength" validator will fail when it shouldn't.

In this PR I'v replaced len with utf8.RuneCountInString. Instead of number of bytes it will calculate number of runes.

It will resolve most issues. But it don't take into account unicode text segmentation. So if there will be symbols such as emojis, that consists of more than one unicode character it still will calculate length incorrectly. More info could be found here.

So may be the next step is to add usage of https://github.com/rivo/uniseg. But considering it is a big change and new dependancy may be it is not worth it.

[danielgtaylor]

@maximdanilchenko thanks for the PR! This one is interesting to me because the length may be used to limit storage in e.g. a DB, so I wonder if we should be counting bytes, runes, or visible characters. It might be useful to give users the choice but that might also be overkill. I think counting runes is probably a good default.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.07%. Comparing base (13acb69) to head (763b669).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #298   +/-   ##
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  Coverage   95.07%   95.07%           
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  Files          19       19           
  Lines        2742     2742           
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  Hits         2607     2607           
  Misses         98       98           
  Partials       37       37           

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[x-user]

IMHO: it's used to validate input string length so it should count runes.