Build/Test Tools: Declare dealerdirect/phpcodesniffer-composer-installer explicitly

[mukeshpanchal27]

Problem

dealerdirect/phpcodesniffer-composer-installer is listed in config.allow-plugins,
but is not in require-dev. It arrives transitively via wp-coding-standards/wpcs and
phpcompatibility/phpcompatibility-wp.

That plugin is what populates the PHP_CodeSniffer installed_paths setting — i.e. whether
composer lint can resolve the WordPress standard at all depends on a package nobody
declared. The project already explicitly trusts it in allow-plugins, so the dependency
is deliberate; only the declaration is missing.

Change

Adds the explicit require-dev entry so the tool that makes linting work is visible.

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Draft proposal from a review of composer.json / package.json. Opened as a draft for discussion — not intended to merge as-is.