Allow configuration of the in-project virtual env directory

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Feature Request

When setting virtualenvs.in-project to true, poetry creates ".venv" directory in the project directory.
When creating in Pycharm a virtual environment for a project, a "venv" directory is created.
It will be very useful if both tools can share the same directory.
To allow this I suggest introducing a new configuration variable called virtualenvs.in-project-venv-name that will be used wherever ".venv" is currently used. The default value for this variable will be ".venv", of course. However, the user will be able to modify it (e.g. to "venv") if necessary.

Start by locating Poetry's handling of the virtualenvs.in-project setting and the code that creates or references the .venv directory. Implement the configurable name while preserving .venv as the default, then verify that a custom name such as venv is used consistently and allows sharing with PyCharm.