[mlir][python] Add pyi stub files to enable auto completion.

There is no completely automated facility for generating stubs that are both accurate and comprehensive for native modules. After some experimentation, I found that MyPy's stubgen does the best at generating correct stubs with a few caveats that are relatively easy to fix:
  * Some types resolve to cross module symbols incorrectly.
  * staticmethod and classmethod signatures seem to always be completely generic and need to be manually provided.
  * It does not generate an __all__ which, from testing, causes namespace pollution to be visible to IDE code completion.

As a first step, I did the following:
  * Ran `stubgen` for `_mlir.ir`, `_mlir.passmanager`, and `_mlirExecutionEngine`.
  * Manually looked for all instances where unnamed arguments were being emitted (i.e. as 'arg0', etc) and updated the C++ side to include names (and re-ran stubgen to get a good initial state).
  * Made/noted a few structural changes to each `pyi` file to make it minimally functional.
  * Added the `pyi` files to the CMake rules so they are installed and visible.

To test, I added a `.env` file to the root of the project with `PYTHONPATH=...` set as per instructions. Then reload the developer window (in VsCode) and verify that completion works for various changes to test cases.

There are still a number of overly generic signatures, but I want to check in this low-touch baseline before iterating on more ambiguous changes. This is already a big improvement.

Differential Revision: https://reviews.llvm.org/D114679

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