Scaffold the module with info file, composer.json and GitLab CI

Problem/Motivation

The project only contains a README with the development roadmap. Every later issue needs a module that Drupal can install and a CI pipeline that runs from the first commit.

Proposed resolution

• Add web_component_runtime.info.yml with core_version_requirement: ^11.
• Add composer.json for drupal/web_component_runtime with type: drupal-module.
• Add the drupal.org GitLab CI template as .gitlab-ci.yml, with the phpcs, phpstan, cspell and PHPUnit jobs enabled.
• Add an empty kernel test so the PHPUnit job has something to run from the first commit.

Remaining tasks

• Add the files above.
• Confirm every CI job passes on the issue fork's pipeline.

User interface changes

None.

API changes

None.

Data model changes

None.