fix(firestore, web): Propagate FirebaseException properly, fix mergeFields bug, fix bytesLoaded different type under different conditions

[russellwheatley]

Description

• Fixed the NativeError that should be FirebaseException. Also - unskipped tests for web testing FirebaseException.
• Fixed bug where we need to Jsify an array with toJS rather than toJSBox.
• Fixed bug where bytesLoaded can be a number or string depending on the circumstances. Not sure if bug with JS SDK or interop.
• Updated some tests were things were not possible on web which now are.
• Improved logic checking FirebaseException.

Related Issues

fixes #12314

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.