feat: validate contracts changeset files

[erikburt]

Validate changesets for contracts.

Changes

• To ensure changeset files that are added to the contracts/.changeset directory, add a new job which validates the changeset files by running pnpm changeset version.

Motivation

Stop files like this from being added. 218a97d was created to fix changeset files. Note:

• The double/single quote changes were not a problem
• The problem is that the changeset in contracts/.changeset had the package chainlink, instead of @chainlink/contracts.

Notes

This is not necessary for core changesets because that is already running a pnpm changeset version for those changes.

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RE-3165