Add `extras` support to `dev` dependencies

[ruben-arts]

Problem description

Currently this dev table will error due to not having support for extras:

[feature.test-deps.dependencies]
scipy = { path = ".", extras = ["test"] }

Resulting in the following error:

Error:   × Unexpected keys, expected only 'url', 'git', 'path', 'branch', 'rev', 'tag', 'subdirectory', 'md5', 'sha256'
     ╭─[/Users/rubenarts/envs/scipy/pixi.toml:313:23]
 312 │ [feature.test-deps.dev]
 313 │ scipy = { path = ".", extras = ["test"] }
     ·                       ───┬──
     ·                          ╰── 'extras' was not expected here
 314 │ 
     ╰────

The path or git or url based definition should support extras, flags and when aswell as dependencies do.

This is useful when you already have a [package] defined and want to use it's configuration instead of duplicating the information in [features.xx tables.