Type Alert_Type, Alert_Trigger, and Exporter families.

[shadyvb]

XWPENG-47 (sub-PR 3 of the typed-properties stack; base: ticket/XWPENG-47-typed-props-connectors).

Adds native string typed properties to the alert and export extension-point families, completing the Rector TypedPropertyFromAssignsRector pass for these classes. Abstract bases (Alert_Type, Alert_Trigger, Exporter) declare $slug / $name as public string with empty-string defaults so child classes can override with concrete values. All seven alert types, three alert triggers, and two exporters receive matching typed $name / $slug (and $meta_key / $field_key on triggers) properties, with redundant @var docblocks removed from the children. rector.php drops the skip rules that deferred this work to PR 3.

No runtime behavior changes — this is a static-analysis and type-safety improvement only.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: N/A — internal typing refactor, no user-facing change.
• New: N/A.

Release Checklist

• This pull request is to the master branch.
• Release version follows semantic versioning. Does it include breaking changes?
• Update changelog in readme.txt.
• Bump version in stream.php.
• Bump Stable tag in readme.txt.
• Bump version in classes/class-plugin.php.
• Draft a release on GitHub.

Change [ ] to [x] to mark the items as done.