[webview_flutter] Opt-in feature that javaScript can automatically open window for iOS platform

[fummicc1]

PR Description

This PR aims to add an option that Javascript can open window without user interaction. related issue is flutter/flutter#112276.

Diving into the detail, I added a setter named setJavaScriptCanOpenWindowsAutomatically to WebViewController that can configure javaScriptCanOpenWindowsAutomatically whose default value is false in iOS platform.

There are two scenarios when javaScriptCanOpenWindowsAutomatically needs to be true.

1. Open window without user interaction such as when loaded html.
2. Open window in asynchronous function such as setInterval. Even with user interaction, iOS prevents webview from opening new window in this scenario.

In general, first scenario should be prevented but second scenario might be accepted in some cases. This is why I tried to solve this issue.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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