Requirements: document iconv as a recommended PHP extension

[rodrigoprimo]

Description

As suggested in squizlabs/PHP_CodeSniffer#3923 (comment), this PR adds iconv to the Requirements wiki page as a strongly recommended PHP extension. Without it, PHP_CodeSniffer falls back to byte-based length calculations, which can cause sniffs like Generic.Files.LineLength to report incorrect results for files containing multibyte characters.

Related issues/external references

Discussed in squizlabs/PHP_CodeSniffer#3923

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the Contribution Guidelines.
• I grant the project the right to include my changes under the BSD-3-Clause license (and I have the right to grant these rights).
• I have verified that my changes comply with the projects coding standards.
• [When adding a new Wiki page] I have added the new page to the _Sidebar.md file.
• [When adding/updating output examples] I have verified via the GitHub Actions artifact that the pre-processing handles the command correctly.

[github-actions]

=== This is an auto-generated comment ===

Thank you for your PR.
A dry-run has been executed on your PR, executing all markdown pre-processing for the wiki files.

Please review the resulting final markdown files via the created artifact.
This is especially important when adding new pages or updating auto-generated output blocks.

N.B.: the above link will automatically be updated when this PR is updated.

[jrfnl]

Thanks @rodrigoprimo !

Potential follow-up:
Would it be an idea to add a suggest section to the composer.json file of the PHPCS package with this recommendation too ?
https://getcomposer.org/doc/04-schema.md#suggest

[jrfnl]

Oh, just thinking as another potential follow-up: may be the pcntl extension should also be added to the "recommended" extensions ? (without this extension, parallel processing doesn't work).

[jrfnl]

Oh and another one as potential follow up related to: PHPCSStandards/PHP_CodeSniffer#294 - have a > [!WARNING] to point out that the gRPC extension may hang PHPCS in combination with parallel processing.