Add interactive pipenv init for creating a Pipfile

[matteius]

Closes #5792

Adds pipenv init, which creates a Pipfile through a short dialog, similar to npm init:

$ pipenv init
Import packages from requirements.txt? [y/n] (y):    # only if one exists
Python version (3.12):
Package index URL (https://pypi.org/simple):
Packages (space-separated, blank for none): requests 'flask[async]>=3'
Dev packages (space-separated, blank for none): pytest
Created …/Pipfile
Create the virtualenv and lock now? [y/n] (y):

Behavior

• -y/--yes accepts every default without prompting (like npm init -y). It's implied when stdin isn't a TTY, so CI never hangs on a prompt.
• --install/--no-install controls whether the virtualenv and lock get created afterwards. By default it asks; under --yes it skips. Installing goes through the normal do_install.
• --python accepts either a bare version (3.12) or an interpreter path.
• It won't overwrite an existing Pipfile in the current directory. A Pipfile in a parent directory doesn't block it: Project() chdirs into the found project's folder at startup, so init switches back to the directory it was invoked from.
• Packages in the dialog are parsed as plain PEP 508 requirements (with shlex, so quoted markers and >=3,<4 both work). Paths, URLs and VCS references are rejected with a note to add them later with pipenv install, since only the full install code builds correct entries for those.

Scope

The issue also asked for name/version/author prompts. Those belong in pyproject.toml, not a Pipfile, so the dialog only covers what a Pipfile records. The docs say so.

Testing

• New tests/unit/test_init_routine.py (16 tests): requirement parsing, --yes with requirements.txt import, bare --python version, refusing an existing Pipfile, the parent-Pipfile case, and the full interactive dialog with mocked prompts.
• Pre-commit (ruff, mypy) passes.
• Tested by hand through a pty: the dialog, rejecting a path package, and the parent-Pipfile case. Also ran pipenv --python 3.12 init -y --install, which imported a requirements.txt, built the venv and wrote Pipfile.lock.

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