feat: #845 用户查看仓库文本文件内容

lib/l10n/intl_en.arb

lib/l10n/intl_zh.arb

...modules/projects/repository/project_repository_model.dart

.../projects/repository/repository_file_content_fetcher.dart

...es/projects/repository/repository_file_content_model.dart

...les/projects/repository/repository_file_content_page.dart

...odules/projects/repository/repository_tree_item_view.dart

lib/modules/projects/repository/repository_tree_model.dart

lib/modules/projects/repository/repository_tree_page.dart

pubspec.lock

pubspec.yaml

...integration_tests/modules/projects/project_page_test.dart

...ntegration_tests/modules/projects/projects_page_test.dart

test/integration_tests/query_issues_by_state_test.dart

...es/projects/repository/project_repository_model_test.dart

...les/projects/repository/project_repository_page_test.dart

...rojects/repository/repository_file_content_page_test.dart

test/modules/projects/repository/response_data.dart

.../modules/projects/starred/projects_starred_page_test.dart