feat: Implement app

app/bin/app.dart

@@ -1,5 +1,11 @@
+import 'dart:io';
+
 import 'package:extract/app.dart' as app;
 
-void main(List<String> arguments) {
-  print('Hello world: ${app.calculate()}!');
+Future<void> main(List<String> arguments) async {
+  final exitCode = await app.runApp(arguments);
+  return Future.wait([
+    stdout.close(),
+    stderr.close(),
+  ]).then((_) => exitCode);
 }

app/lib/app.dart

@@ -1,3 +1,65 @@
-int calculate() {
-  return 6 * 7;
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:extract/src/app_exeption.dart';
+import 'package:extract/src/project_config.dart';
+import 'package:file/local.dart';
+import 'package:recase/recase.dart';
+
+Future<int> runApp(List<String> arguments) async {
+  final path = arguments.firstOrNull;
+  if (path == null) {
+    stderr.writeln('Usage: extract <path>');
+    return 1;
+  }
+
+  try {
+    await _extract(path);
+    stdout.writeln(r'Successfully extracted project config to $GITHUB_OUTPUT');
+    return 0;
+  } on AppException catch (e) {
+    stderr.writeln(e.message);
+    return 1;
+  }
+}
+
+Future<void> _extract(String path) async {
+  final fileSystem = LocalFileSystem();
+  final projectConfigFile = fileSystem.file(path);
+  if (!projectConfigFile.existsSync()) {
+    throw AppException('Project config file not found at $path');
+  }
+
+  final projectConfigContent = projectConfigFile.readAsStringSync();
+
+  final Map<String, dynamic> json;
+  try {
+    json = jsonDecode(projectConfigContent) as Map<String, dynamic>;
+  } on FormatException catch (e) {
+    throw AppException('Invalid project config file at $path: $e');
+  }
+
+  final ProjectConfig projectConfig;
+  try {
+    projectConfig = ProjectConfig.fromJson(json);
+  } on FormatException catch (e) {
+    throw AppException('Invalid project config file at $path: $e');
+  }
+
+  final projectConfigJson = projectConfig.toJson();
+  final futures = projectConfigJson.entries.map(
+    (entry) => Process.run(
+      'echo',
+      [
+        '"${entry.key.paramCase}=${entry.value}"',
+        '>>',
+        r'$GITHUB_OUTPUT',
+      ],
+    ),
+  );
+  final results = await Future.wait(futures);
+  final exitCodes = results.map((result) => result.exitCode);
+  if (exitCodes.any((exitCode) => exitCode != 0)) {
+    throw AppException(r'Failed to write project config to $GITHUB_OUTPUT');
+  }
 }