Refactor js-yaml import usage

[Sefirost]

Hi! I've open this simple PR to adjust the import usage of js-yaml that provides the yaml schema. It consist of three changes:

1. In order to maintain project consistency, use import instead of require. It also prevents this issue when the code is loaded as ESM.
   

2. Add the types for js-yaml as dev dependency.

3. Since js-yaml is now imported, it needs to be marked as external dependency for rollup to avoid this warning:
   

[github-actions]

🧪 A prerelease is available for testing 🧪

You can install this latest build in your project with:

npm install --save https://prerelease-registry.devprod.cloudflare.dev/itty-router-openapi/runs/8174772066/npm-package-itty-router-openapi-131

Or you can immediately run this with npx:

npx https://prerelease-registry.devprod.cloudflare.dev/itty-router-openapi/runs/8174772066/npm-package-itty-router-openapi-131

[G4brym]

Thanks for this fix @Sefirost