Feat/feature 2 - Add final progress to go learning process

• add struct.go
• add pointer.go
• add interface.go
• add nil.go
• add type_assertion.go
• add package helper and import.go
• add go.mod
• add access_modifier.go
• add package_initialization.go
• add blank_identifier.go
• add errors_basic.go
• add errors_custom.go