feat: validate "melos exec" is not used in "run" along with "exec"

[BenVercammen]

Description

As described in #437, when using melos exec in scripts/*/run while also defining scripts/*/exec, an exception should be thrown as this would lead to melos exec being executed in every package.

Type of Change

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore

[blaugold]

@BenVercammen Thanks for the contribution! I refactored the validation to run when the options in melos.yaml are validated to give users this feedback as early as possible.