[camera_web] Migrate to package:web

[Rexios80]

Fixes flutter/flutter#139748

Notes:

• Some fields are no longer nullable in package:web. We need to decide if we want to make them nullable again.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• 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.)
• I signed the CLA.
• 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.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• 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.

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[Rexios80]

I figured out how to run the tests compiled to WASM. Working on resolving the issues.

[Rexios80]

Alright I think all the tests pass when compiled to WASM now

[Rexios80]

The example app is having this issue: flutter/flutter#146539

[ditman]

I figured out how to run the tests compiled to WASM. Working on resolving the issues.

@Rexios80 Please describe? :)

[Rexios80]

@ditman it's just flutter drive -d chrome --wasm

Probably requires master channel