[MLIR][Python] reland (narrower) type stub generation (#157930)

This a reland of https://github.com/llvm/llvm-project/pull/155741 which
was reverted at https://github.com/llvm/llvm-project/pull/157831. This
version is narrower in scope - it only turns on automatic stub
generation for `MLIRPythonExtension.Core._mlir` and **does not do
anything automatically**. Specifically, the only CMake code added to
`AddMLIRPython.cmake` is the `mlir_generate_type_stubs` function which
is then used only in a manual way. The API for
`mlir_generate_type_stubs` is:

```
Arguments:
  MODULE_NAME: The fully-qualified name of the extension module (used for importing in python).
  DEPENDS_TARGETS: List of targets these type stubs depend on being built; usually corresponding to the
    specific extension module (e.g., something like StandalonePythonModules.extension._standaloneDialectsNanobind.dso)
    and the core bindings extension module (e.g., something like StandalonePythonModules.extension._mlir.dso).
  OUTPUT_DIR: The root output directory to emit the type stubs into.
  OUTPUTS: List of expected outputs.
  DEPENDS_TARGET_SRC_DEPS: List of cpp sources for extension library (for generating a DEPFILE).
  IMPORT_PATHS: List of paths to add to PYTHONPATH for stubgen.
  PATTERN_FILE: (Optional) Pattern file (see https://nanobind.readthedocs.io/en/latest/typing.html#pattern-files).
Outputs:
  NB_STUBGEN_CUSTOM_TARGET: The target corresponding to generation which other targets can depend on.
```

Downstream users should use `mlir_generate_type_stubs` in coordination
with `declare_mlir_python_sources` to turn on stub generation for their
own downstream dialect extensions and upstream dialect extensions if
they so choose. Standalone example shows an example.

Note, downstream will also need to set
`-DMLIR_PYTHON_PACKAGE_PREFIX=...` correctly for their bindings.