[webview_flutter] Complete move to federated architecture

[BeMacized]

This PR removes the native implementations that have been moved to their own separate packages in #4315, and contains the necessary refactors to complete the move to a federated architecture.

As this PR still relies on the platform interface package being merged and published (#4302), as well as the native Android and iOS implementations being merged and published (#4315), it will remain in draft state until those dependencies are satisfied, so that this PR may be updated accordingly.

Relevant issue:

• [webview_flutter] Migrate plugin to federated architecture flutter#86286

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[mvanbeusekom]

@googlebot I consent.

[mvanbeusekom]

Closing this pull request, as I will be submitting new PRs where the addition of Android and iOS platform support will be separated. I will add @BeMacized as Co-author to these PRs.