feat: Implement PEP735 support

[olivier-lacroix]

Fixes #2256 as well as #2149 (maybe we want to create a separate issue to interpret / use markers).

This PR adds handling of [dependency-groups] in pyproject.toml:

• any group in this table is interpreted as a feature by pixi (similar to current behaviour for optional-dependencies)
• if a dependency group and a group of optional dependencies have the same name, all dependencies will be added to a single common feature

This PR also changes the behaviour of pixi add --pypi in case of a pyproject.toml manifest:

• if a feature is specified, the dependency is now added to the [dependency-groups] table instead of the [optional-dependencies] table currently
• if --platform or --editable is specified, the pypi dependency will be added to the tool.pixi.pypi-dependencies table instead, since native arrays have no support for platform-specific or editable dependencies.

This PR also changes the behaviour of pixi init when starting from an existing pyproject.toml manifest:

• one environment per dependency-group will be added to the environments table, similar to what is being done currently with groups of optional dependencies.

As a drive-by improvement, when removing a pypi dependency from a pyproject.toml manifest, any empty array will be deleted.

[ruben-arts]

First off, thank you 💯, amazing contribution! Well documented, Kudos!

I have a few small requests to finish it off.

Also could it print in what location it was added in the pixi add output. e.g.:

❯ pixi add --pypi --location optional-dependencies boltons
✔ Added boltons >=24.1.0, <25
Added these to `optional-dependencies`.

❯ pixi add --pypi numpy
✔ Added numpy >=2.1.3, <3
Added these as `project.dependencies`.

❯ pixi add --pypi --feature test pytest
✔ Added numpy >=2.1.3, <3
Added these as `dependency-groups`.

[tdejager]

@olivier-lacroix would having a dependency-group with foo and an optional-feature with foo merge the dependencies together into the same foo pixi feature?

[olivier-lacroix]

@olivier-lacroix would having a dependency-group with foo and an optional-feature with foo merge the dependencies together into the same foo pixi feature?

Yes it would

[olivier-lacroix]

@ruben-arts , @tdejager this is ready to merge I think.

@ruben-arts I started implementing the location printing, but I think this can wait a follow-up PR / decision on the CLI API.

[ruben-arts]

Superb! Thanks for implementing this!