fix: js-yaml is actually a dependency

[Shywim]

Move js-yaml from devDependencies to dependencies, as this is actually used in the library code and this triggers some package managers such as yarn v2+:

❯ yarn serverless deploy
Environment: darwin, node 18.10.0, framework 3.23.0 (local), plugin 6.2.2, SDK 4.3.2
Docs:        docs.serverless.com
Support:     forum.serverless.com
Bugs:        github.com/serverless/serverless/issues

Error:
Error: serverless-scaleway-functions tried to access js-yaml, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.

[thomas-tacquet]

Thank you for your contribution, it will be included in the next release 👌