[REFACTOR][STUBGEN] refactor the stubgen logic. (#608)

## Summary

This PR refactors the Python stubgen implementation to improve
extensibility and separation of concerns. No functional changes are
intended, and the generated output should remain unchanged.

## Design

The current stubgen implementation contains both language-independent
logic (e.g., file processing and library analysis) and language-specific
code generation. This PR separates these responsibilities by introducing
a generator abstraction for the code generation layer, making it easier
to support additional target languages in the future.

The reorganized file structure is:

```text
python/tvm_ffi/stub/
├── __init__.py
├── cli.py
├── consts.py
├── file_utils.py
├── lib_state.py
├── utils.py
├── generator.py
└── python_generator/
    ├── __init__.py
    ├── generator.py
    ├── codegen.py
    ├── consts.py
    └── utils.py
```

This refactoring lays the groundwork for future support of Rust stub
generation and other language backends.

## Testing

This PR is a pure refactor and is not expected to change existing
behavior. Therefore, no new tests are added.

Signed-off-by: yuchuan <yuchuan.7streams@gmail.com>