Mark module_extension or tag_class deprecated

### Description of the feature request:

Add a flag to mark module extensions or single tag classes deprecated and print a warning to the user when used.

### Which category does this issue belong to?

Core

### What underlying problem are you trying to solve with this feature?

Help users migrate away from deprecated features.

### Which operating system are you running Bazel on?

_No response_

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

The issue names module_extension and single tag classes but does not identify files, tests, or an entry point. Begin by tracing where these features are declared and used, then clarify the warning behavior and scope before implementation. Done means deprecated declarations can be marked and usage produces a user-visible warning.

Written by the indexing model from the issue text.