Added support for script chaining and env vars

[mitsuhiko]

This adds support for script chaining and environment variables like so:

[tool.rye.scripts]
serve = { cmd = "flask run", env = {"FLASK_APP" = "./hello.py"} }
lint = { chain = ["lint:black", "lint:flake8" ] }
"lint:black" = "black --check src"
"lint:flake8" = "flake8 src"

With this you can run rye run lint to invoke first rye run lint:black followed by rye run lint:flake8 if the first script did not fail. This really needs docs.

Output in list:

lint (chain: [lint:black], [lint:flake8])
lint:black (black --check src)
lint:flake8 (flake8 src)
serve (FLASK_APP=./hello.py flask run)

Fixes #152