feat: Multer limit option validation and error messages

[ShubhamOulkar]

Task List

• Validate all limits options using checkLimitValue
• Throw MulterError with detailed context for invalid limit values
• Ensure getLimits only accepts plain objects and validates all keys
• Improve error reporting and context for easier debugging
• documentation (if needed)

closes #1132

[coveralls]

coverage: 98.909% (+0.4%) from 98.485%
when pulling f486b10 on ShubhamOulkar:busboy-limits
into e259a7e on expressjs:main.