fix: Add characterCountText to form field for improved screen reader support

[avinashbot]

Description

Apparently, NVDA/JAWS don't fare well when you update the accessible description of an input you're focused on after every keypress, and end up interrupting everything else, like the keypress announcements themselves.

This introduces a new string property, characterCountText, whose updates are debounced for screen readers. This also means that only the updated section of the constraint text ("Character count: x/y") is announced when it changes.

Related links, issue #, if available: AWSUI-61742

How has this been tested?

Added unit tests and a test util method.

Provisioning a Windows machine right now, but I'll also let our accessibility specialists take a crack at it before merging. But in the meantime, the code itself is reviewable.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (f7f12ee) to head (d0adbdd).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4332   +/-   ##
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  Coverage   97.43%   97.43%           
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  Files         897      897           
  Lines       26342    26357   +15     
  Branches     9517     9522    +5     
=======================================
+ Hits        25667    25682   +15     
  Misses        632      632           
  Partials       43       43           

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