Adding a Swift-to-Dart guide (#4427)

Staged:
https://sz-www-1.web.app/guides/language/coming-from/swift-to-dart

@kenzieschmoll @leighajarett @hellohuanlin @parlough

Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: sfshaza2

.firebaserc

@@ -4,11 +4,6 @@
     "dev": "dartlang-org-dev",
     "staging-0": "dart-dev-staging-0",
     "staging-1": "dart-dev-staging-1",
-    "staging-0-old": "dartlang-org-staging-0",
-    "staging-1-old": "dartlang-org-staging-1",
-    "kw": "kw-www-dartlang-1",
-    "kw2": "kw-staging-dartlang-2",
-    "kw3": "kw-dartlang-3",
     "sz": "sz-www-1",
     "sz2": "sz-www-2",
     "jr": "jr-dartlang-site"

src/_data/side-nav.yml

@@ -50,6 +50,8 @@
       children:
         - title: JavaScript to Dart
           permalink: /guides/language/coming-from/js-to-dart
+        - title: Swift to Dart
+          permalink: /guides/language/coming-from/swift-to-dart
     - title: Extension methods
       permalink: /guides/language/extension-methods
     - title: Concurrency