Ensure that the MANIFEST.in file includes all necessary folders

[huangzhii]

Minor improvement: Add MANIFEST.in

The MANIFEST.in file is used to include additional files and directories in the source distribution that might not be automatically included by setuptools.

For example, without MANIFEST.in, when install textgrad via

pip install git+https://github.com/zou-group/textgrad.git

and then run import textgrad, the terminal will throw below error:

>>> import textgrad
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/anaconda3/lib/python3.9/site-packages/textgrad/__init__.py", line 20, in <module>
    from .variable import Variable
  File "/opt/anaconda3/lib/python3.9/site-packages/textgrad/variable.py", line 8, in <module>
    from .utils.image_utils import is_valid_url
ModuleNotFoundError: No module named 'textgrad.utils'

Therefore, we need to include the utils/ folder in the MANIFEST.in file (in this case, recursive-include textgrad *), so that this folder can be packaged and installed correctly.