packages [google_adsense] AdSense plugin for Flutter Web

Google Adsense plugin should allow Flutter Web developers to integrate Adsense more easily and monetize web project.

List which issues are fixed by this PR. You must list at least one issue. https://github.com/flutter/flutter/issues/40376

Pre-launch Checklist

[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[ ] I read the Tree Hygiene page, which explains my responsibilities.
[x] 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.)
[x] I signed the CLA.
[x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
[x] I linked to at least one issue that this PR fixes in the description above.
[x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
[x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
[x] 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.