[Sync EN] filter: Refactor filter constants by splitting into separate files

[github-actions]

PR: https://github.com/php/doc-en/pull/5886 (2026-09-27)

Файлы RU для обновления

• reference/filter/constants.xml

Новые файлы EN (ещё не переведены)

• reference/filter/constants.callback.xml
• reference/filter/constants.flags.generic.xml
• reference/filter/constants.flags.sanitization.xml
• reference/filter/constants.input.xml
• reference/filter/constants.sanitization.xml
• reference/filter/constants.validation.xml