warn when `netlify.toml` uses `yarn` but repo only has package-lock.json

[swyxio]

- Do you want to request a feature or report a bug?
feature

- What is the current behavior?
if the repo only has package-lock.json, we do not install yarn, however the user's netlify.toml may use yarn. this will cause a bug we can catch earlier before a commit or a deploy.

this is a common user error. i do it.