Add GoModule support to template context and update gitignore

- Add GoModule struct with Module and Version fields
- Parse go.mod file to extract module name and Go version
- Expose GoModule in template context as {{.GoModule}}
- Update asset templates to use {{.GoModule}} instead of hardcoded values
- Add gitignore pattern for testdata directories (keep only plugin.json files)
- All templates now have access to both manifest and Go module information

Templates can now use:
- {{.GoModule.Module}} for module name
- {{.GoModule.Version}} for Go version
- {{if .GoModule}}...{{end}} for conditional logic

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

assets/build/utils.mk

@@ -33,7 +33,7 @@ endif
 mock:
 ifneq ($(HAS_SERVER),)
 	go install github.com/golang/mock/mockgen@v1.6.0
-	mockgen -destination=server/command/mocks/mock_commands.go -package=mocks github.com/mattermost/mattermost-plugin-starter-template/server/command Command
+	mockgen -destination=server/command/mocks/mock_commands.go -package=mocks {{.GoModule}}/server/command Command
 endif
 
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