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/.golangci.yml

@@ -6,7 +6,7 @@ linters-settings:
   gofmt:
     simplify: true
   goimports:
-    local-prefixes: github.com/mattermost/mattermost-starter-template
+    local-prefixes: {{.GoModule}}
   govet:
     check-shadowing: true
     enable-all: true