feat: inline environments

[Hofer-Julian]

Description

Not to be confused with inline packages, this PR implements the ability to do this:

[environments.dev.dependencies]
git = "*"

[environments.test.dependencies]
pytest = "*"
pytest-xdist = "*"

# features can still be added
[feature.python.dependencies]
python = "3.14.*"

[environments]
dev = { features = ["python"] }
test = { features = ["python"] }

Fixes #5317

How Has This Been Tested?

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Claude

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.