[sidlatau]

Adds an optional env field to setup_project configurations, applied only to that configuration's run/debug process.

• Job runner: merged on top of Neovim's environment, since plenary's Job replaces the whole environment when env is set.
• Debugger runner: merged into the launch configuration's env; the Dart/Flutter debug adapters already add it to their own inherited environment.
• Values are converted with tostring, so env = { RETRIES = 3 } works like dart_define.

Closes #562