Draft: MergeMessageFileLists.php: Use a realm-specific extension-list

Our guidelines for "Writing an extension for deployment"0 have "deploy to Beta Cluster" as a pre-production step.

The instructions say to add your extension to our release branching tool, which means the extension will be on-disk in production for two weeks before deployment to beta.

The only reason to do this oddity, is because you need to add your extension to extension-list in operations/mediawiki-config to generate l10n in beta.

This is a problem because that file is shared between production and beta.

After this change, beta and production no longer have to share a single extension-list file, an extension-list-labs file can be used instead.

The future process for releasing an extension then, would be to add your extension to the extension-list-labs file, and add it as a submodule in the mediawiki/extensions project. No need to wait two weeks and no need to deploy to production before beta.

Removing FIXME for get_realm_specific_filename extensionless files returning the base filename. All existing uses of this function seem to have extensions, so this should be a noop.

Bug: T125678