Update flutter SDK supported platforms list

pub.dev incorrectly shows that the appwrite flutter SDK doesn't support
web when it does. This overrides what pub.dev detects to explicitly
include web as supported.

Author: stnguyen90

templates/flutter/pubspec.yaml.twig

@@ -5,6 +5,13 @@ homepage: {{sdk.url}}
 repository: https://github.com/{{sdk.gitUserName}}/{{sdk.gitRepoName}}
 issue_tracker: https://github.com/appwrite/sdk-generator/issues
 documentation: {{ spec.contactURL }}
+platforms:
+  android:
+  ios:
+  linux:
+  macos:
+  web:
+  windows:
 environment:
   sdk: ">=2.17.0 <3.0.0"