[rfw] Add web implementation for the rfw wasm example

[xster]

Fixes https://github.com/flutter/flutter/issues/129384.

• Slightly updated the readme section
• Split networking and wasm implementations into 2 files since the upstream APIs for desktop and web are very different
• I left the binary encoding bits for rfw, but since dart2js doesn't support 64 bit int, I did the demo in dart2wasm web
• I removed the local caching bit. It's probably a distraction from the core of the demo. Since the web implementation will need to work very differently, I think it distracts from the main demo.

Pre-launch Checklist

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• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
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• [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 listed 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.
• [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.
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[flutter-dashboard[bot]]

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[Hixie]

Sorry it took me forever to follow up, I totally forgot about this PR. Please ping me when you want it reviewed again, I get too much GitHub notification spam to be able to rely on that.

[xster]

I synced this again since stable is out now. PTAL

[stuartmorgan-g]

Merged in main now that stable has been updated.

[stuartmorgan-g]

@xster Are you still planning on updating the PR to make it landable?