This PR adds a `deprecated_module` command that marks the current module
as deprecated. When another module imports a deprecated module, a
warning is emitted during elaboration suggesting replacement imports.
Example usage:
```lean
deprecated_module "use NewModule instead" (since := "2026-03-30")
```
The warning message is optional but recommended. The `since` parameter
is required. Warnings can be disabled, by setting
`linter.deprecated.module` option to false in the command line. Because
the check happens when importing , using `set_option
linter.deprecated.module` in the source file won't affect the warnings.
Instead, a whole file can be marked not to display depreciation
warnings, by putting a comment `deprecated_module: ignore` next to
`module` keyword. Similarly, individual keywords can be silenced.
A `#show_deprecated_modules` command is also provided for inspecting
which modules in the current environment are deprecated.
`linter.deprecated.module` has no effect on this command, and hence one
can view deprecated modules, even when having warnings silenced.
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