Add dependabot configuration file

[jrfnl]

Proposed Changes

This automatically enables Dependabot to:

• Submit pull requests for security updates and version updates for Composer dependencies.
• Submit pull requests for security updates and version updates for GH Action runner dependencies.

For Composer dependencies, a preference is given to widen the version restrictions instead of updating them to a new minimum. This is a deliberate choice as this package is a library, not an application.

The configuration has been set up to:

• Run once a week.
• Submit a maximum of 5 pull requests at a time. If additional pull requests are needed, these will subsequently be submitted the next time Dependabot runs after one or more of the open pull requests have been merged.
• The commit messages for PRs submitted by Dependabot will be prefixed according the unofficial conventions used in this repo up to now.
• The PRs will automatically be labelled with an appropriate label as already in use in this repo.

Additionally, for Composer updates, I've applied the following restrictions:

• Only allow updates for "dev" dependencies, as non-dev dependencies (PHPCS, Composer Installers) will need a code review and likely warrant code changes.
• Ignore major releases of the PHPUnit Polyfills package (= new PHPUnit major) as those generally require a managed update of the test suite.

Refs:

• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#versioning-strategy