[messages] Use camelCase in messages.yaml files.

Updates the `messages.yaml` files to use `camelCase` naming
conventions for message keys, `sharedName` fields, and `analyzerCode`
fields. This eliminates a significant inconsistency between the
diagnostic code formats in the analyzer and the front end.

This CL was created automatically by running the script
`pkg/analyzer_utilities/tool/messages/switch_to_camel_case.dart`.

Change-Id: I6a6a6964cc9e5b2ff9d09875d49f0f3d3f404121
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467060
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441