chore: reorganize snippets into subdirectories

- move root gitignores into gitignores/ subdirectory
- add gitignores for dart, dotnet, go, gradle, java, jetbrains, maven, nextjs, node, sublimetext, vscode, windows, xcode
- add root .gitignore excluding .claude/settings.local.json
- add empty README.md and licences/ placeholders (Apache, MIT, Mozilla)

gitignores/Dart.gitignore

@@ -0,0 +1,28 @@
+### Dart
+# Files and directories created by pub
+.dart_tool/
+.packages
+build/
+# If you're building an application, you may want to check-in your pubspec.lock
+pubspec.lock
+
+# Directory created by dartdoc
+# If you don't generate documentation locally you can remove this line.
+doc/api/
+
+# dotenv environment variables file
+.env*
+
+# Avoid committing generated Javascript files:
+*.dart.js
+# Produced by the --dump-info flag.
+*.info.json
+# When generated by dart2js. Don't specify *.js if your
+# project includes source files written in JavaScript.
+*.js
+*.js_
+*.js.deps
+*.js.map
+
+.flutter-plugins
+.flutter-plugins-dependencies