Fixed and improvements in is_LLL_reduced and approximate_closest_vector

[TheBlupper]

This fixes #38115. Along with the goal stated there I have fixed some stuff I wasn't happy with from the first iteration of approximate_closest_vector. These changes could be breaking but seeing as there has been no release since its introduction this shouldn't be a problem.

Changes:

• Added the algorithm parameter to is_LLL_reduced along with the new algorithm fpLLL which uses fpylll's LLL.is_reduced method, and made it the default, as I see no advantages with the old implementation.
• Added the algorithm parameter to approximate_closest_vector along with the new algorithms embedding and rounding_off, and made embedding the default.
• Changed the alias of approximate_closest_vector from babai to cvp to be more accurate but still short.

I'm getting weird errors when building documentation locally but I don't think it's related to my changes, so I haven't been able to check if the doc build looks alright yet. (and it won't build on my own repo b/c my GitHub username has uppercase letters in it :)

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

[github-actions]

Documentation preview for this PR (built with commit 605c02a; changes) is ready! 🎉
This preview will update shortly after each push to this PR.

[mkoeppe]

Lint failures

[TheBlupper]

The code coverage that's missing feels a bit silly, should I still create tests for them? (one of them is unreachable so idk)

[GiacomoPope]

Small comment about comment formatting, otherwise good