fix: add go.mod, fix test example names and printf format warning

[aoright]

Problem

1. In modern Go toolchains (go test / go vet), difflib_test.go fails to compile because example function names (ExampleGetUnifiedDiffCode, ExampleGetContextDiffCode) reference unknown identifiers, and fmt.Printf is called with a dynamic format string.
2. Lack of go.mod causes module-aware tooling to require GOPATH mode.

Fix

• Add go.mod definition for github.com/pmezard/go-difflib.
• Rename ExampleGetUnifiedDiffCode to ExampleGetUnifiedDiffString and ExampleGetContextDiffCode to ExampleGetContextDiffString_format.
• Replace fmt.Printf with fmt.Print in ExampleGetContextDiffString.