fix(wkwebview): make webview first responder, closes #739

[rhysd]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

I confirmed #739 was solved on my local machine (macOS 11).

Just after launching the application window, typing some keys is reflected:

[rhysd]

I forgot committing a .changes file. I amended and force-pushed.