Add GitHub issue templates

[pronskiy]

Create .github/ISSUE_TEMPLATE/ with:

• bug_report.yml — PHP version, OS, extension version, php.ini config, steps to reproduce, expected vs actual behavior
• feature_request.yml — Description, use case, proposed solution
• config.yml — Link to discussions for questions

Use YAML form-based templates (not markdown).

Acceptance criteria

• Bug report template collects PHP version, OS, extension version, steps to reproduce
• Feature request template exists
• "New Issue" page shows template chooser