Remove yarn dependency

[mikemaccana]

• Changes current behavior (require warn, try yarn, warn if yarn is missing, use npm) to just using npm
• Use npm in test scripts too
• Update 'mismatched version' error message to also use npm

This PR doesn't change yarn being used to build Anchor itself.

Addresses #2881

[vercel]

@mikemaccana is attempting to deploy a commit to the coral-xyz Team on Vercel.

A member of the Team first needs to authorize it.

[acheroncrypto]

I think setting the default to npm is great, but people might still want to use yarn. I think we should give that optionality as a flag in necessary commands.

[mikemaccana]

(will do, I just need to get around to it)