feat: Initial package and CI setup

[scottmarchant]

Summary

Sets up initial swift package and CI setup.

Details

• de8158c build: Initial commit of swift package init with package name nio-async-runtime and target/library name NIOAsyncRuntime
• 612c87b build: Add dependencies used by NIOAsyncRuntime.
• 73769e2 ci: Set up basic pull request CI using https://github.com/PassiveLogic/swift-dispatch-async/blob/main/.github/workflows/pull_request.yml as a source of inspiration.

Testing Performed

• Verified package resolves and builds locally and in CI

Overview of all changes

All planned changes can be viewed together in #2