permit setting/appending structures

[blast-hardcheese]

Hello! Thank you for the tool, I noticed it wasn't possible to fully construct new complex structures.

I figured this could be done incrementally with minimal effort, using {} and [] as literal symbols bound to specific functionality:

$ tomato set    'tool.uv.sources.torch'          '[]'          pyproject.toml
$ tomato append 'tool.uv.sources.torch'          '{}'          pyproject.toml
$ tomato set    'tool.uv.sources.torch[0].index' 'pytorch-cpu' pyproject.toml

when applied to the following toml:

[project]
name = "beep"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = []

[tool.uv.sources]
torchvision = [
    { index = "pytorch-cpu", marker = "platform_system == 'Linux'" },
]

results in

[project]
name = "beep"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "requests>=2.32.3"
]

[tool.uv.sources]
torchvision = [
    { index = "pytorch-cpu", marker = "platform_system == 'Linux'" },
]
torch = [{ index = "pytorch-cpu" }]