Add option to include docstrings with stubgen (#13284)
### Description
Closes #11965.
Add a --include-docstrings flag to stubgen. This was suggested in #11965
along with a use case.
When using this flag, the .pyi files will include docstrings for Python
classes and functions and for C extension functions.
The flag is optional and does not change the default stubgen behaviour.
When using the flag, the resulting function stubs that contain docstring
will no longer be one-liners, but functions without a docstring still
retain the default one-liner style.
Example input:
```python
class A:
"""class docstring"""
def func():
"""func docstring"""
...
def nodoc():
...
```
output:
```python
class A:
"""class docstring"""
def func() -> None:
"""func docstring"""
...
def nodoc() -> None: ...
```
## Test Plan
Tests `testIncludeDocstrings` and `testIgnoreDocstrings` were added to
`test-data/unit/stubgen.test` to ensure the code works as intended. All
other tests passed as well.
C extension docstrings are tested using an updated bash script
`misc/test_stubgenc.sh` with test data in
`test-data/pybind11_mypy_demo/stubgen-include-docs` in same fashion as
in an already existing test.
---------
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
