feat: add library setup for linux

bin/setup.dart

@@ -0,0 +1,100 @@
+import 'dart:io';
+
+import 'package:archive/archive.dart';
+import 'package:args/command_runner.dart';
+import 'package:cli_util/cli_logging.dart' show Ansi, Logger;
+import 'package:libgit2dart/src/util.dart';
+
+bool libgit2IsPresent(String platform) {
+  final result = File.fromUri(
+    Directory.current.uri
+        .resolve('.dart_tool/libgit2/$platform/${getLibName(platform)}'),
+  ).existsSync();
+  return result;
+}
+
+/// Extracts a tar.gz file.
+void extract(String fileName, String dir) {
+  final tarGzFile = File(fileName).readAsBytesSync();
+  final archive = GZipDecoder().decodeBytes(tarGzFile, verify: true);
+  final tarData = TarDecoder().decodeBytes(archive, verify: true);
+  for (final file in tarData) {
+    File('$dir${file.name}')
+      ..createSync(recursive: true)
+      ..writeAsBytesSync(file.content as List<int>);
+  }
+}
+
+/// Downloads libgit2 from GitHub releases, extracts and places it in correct
+/// directory.
+Future<void> download(String platform) async {
+  final logger = Logger.standard();
+  final ansi = Ansi(Ansi.terminalSupportsAnsi);
+
+  if (libgit2IsPresent(platform)) {
+    if (libgit2Version == getVersionNumber()) {
+      logger.stdout('${ansi.green}libgit2 for $platform is already available.');
+    } else {
+      logger.stdout(
+        '${ansi.red}libgit2 for $platform is outdated. Run: \n'
+        'flutter pub run libgit2dart:setup clean\n'
+        'flutter pub run libgit2dart:setup',
+      );
+    }
+  } else {
+    final fileName = '$platform.tar.gz';
+    final downloadUrl = '$libUrl$fileName';
+    logger.stdout('Downloading libgit2 for $platform');
+    logger.stdout(downloadUrl);
+
+    try {
+      final httpClient = HttpClient();
+      final request = await httpClient.getUrl(Uri.parse(downloadUrl));
+      final response = await request.close();
+      final fileSink = File(fileName).openWrite();
+      await response.pipe(fileSink);
+      await fileSink.flush();
+      await fileSink.close();
+      httpClient.close();
+    } catch (error) {
+      Exception("Can't download. Check your internet connection.");
+    }
+
+    logger.stdout('${ansi.yellow}Extracting libgit2 for $platform${ansi.none}');
+    Directory('$libDir$platform/').createSync(recursive: true);
+    extract(fileName, '$libDir$platform/');
+    logger.stdout('${ansi.green}Done! Cleaning up...');
+
+    File(fileName).deleteSync();
+
+    logger.stdout(
+      '${ansi.green}Done! libgit2 for $platform is now available!'
+      '${ansi.none}',
+    );
+  }
+}
+
+class CleanCommand extends Command<void> {
+  @override
+  String get description => 'Cleans downloaded libraries.';
+
+  @override
+  String get name => 'clean';
+
+  @override
+  void run() {
+    final logger = Logger.standard();
+    logger.stdout('cleaning...');
+    Directory(libDir).deleteSync(recursive: true);
+  }
+}
+
+void main(List<String> args) {
+  final runner = CommandRunner<void>(
+    'setup',
+    'Downloads the libgit2 library.',
+  );
+  runner.addCommand(CleanCommand());
+
+  (args.isEmpty) ? download(Platform.operatingSystem) : runner.run(args);
+}