Add export command extract alternative lockfile format

[manzt]

Adds export command to extract alternative lockfile format (i.e., requirements.txt-style) to stdout:

juv init
juv add Untitled.ipynb attrs
juv export Untitled.ipynb
# # This file was autogenerated by uv via the following command:
# #    uv export --script /Users/manzt/github/manzt/juv/tmp54f6swns.py
# attrs==24.3.0 \
#     --hash=sha256:8f5c07333d543103541ba7be0e2ce16eeee8130cb0b3f9238ab904ce1e85baff \
#     --hash=sha256:ac96cd038792094f438ad1f6ff80837353805ac950cd2aa0e0625ef19850c308