Insert `pyproject.toml` `[dependency-groups]` entries in sorted order

[woutervh]

Summary

> uv --version
uv 0.5.30

create a new package, and add some dependencies in various dependency-groups:

uv init demo --package    
cd demo
uv add pytest --group testing        
uv add mypy --group typing   
uv add ruff --group dev  

This leads to ordering in the way they were added:

[dependency-groups]
testing = [
    "pytest>=8.3.4",
]
typing = [
    "mypy>=1.15.0",
]
dev = [
    "ruff>=0.9.6",
]

cleaner would be to order the keys:

[dependency-groups]
dev = [
    "ruff>=0.9.6",
]
testing = [
    "pytest>=8.3.4",
]
typing = [
    "mypy>=1.15.0",
]

Example

No response

[zanieb]

Seems reasonable to do what we do for dependencies — insertions should be sorted if the list is already sorted. Otherwise, they should not to avoid churn.