fix(components): [input] unify async-validator string length calc

[btea]

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fix #16756

The internal string length calculation method of async-validator is inconsistent with input. In order to keep the results consistent, the calculation methods of the two are unified.

Since async-validator has not been updated for two years, the fastest, most efficient and controllable way is to generate patch file modifications, so I made this modification.

[github-actions]

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=16757
Please comment the example via this playground if needed.

[makedopamine]

Question: Why do you choose the length property which counts the length of a string in UTF-16 code units? Why not count the length in code points?

[btea]

Follow the calculation method of the tag's native maxlength attribute.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/textarea#maxlength

[makedopamine]

Follow the calculation method of the tag's native maxlength attribute. https://developer.mozilla.org/en-US/docs/Web/HTML/Element/textarea#maxlength

Well, that makes sense.

[plainheart]

Ref: #6588 #6127

[btea]

@kooriookami I think the spelling errors in the original comments may not need to be corrected. 😀