[google_adsense_ad_placement_api_web] Creates new package to simplify interaction with AdSense Ad Placement API for Flutter Apps

[astivi]

Official library release to simplify monetization of Flutter Apps by providing an easy to use interface to the AdSense Ad Placement API in the Web platform. Please note that this package only supports interacting with the Ad Placement API after it's already loaded. Before using it remember to include the appropriate adsense js files.

List which issues are fixed by this PR. You must list at least one issue.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] 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]
• [ ] 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 ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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