Support env variables in project configurations #562

Today there is no way to set environment variables for the flutter run command per project configuration. The process started by FlutterRun inherits Neovim's environment, so env vars have to be exported in the shell before starting Neovim, or set with vim.env in pre_run_callback. The vim.env approach leaks the value into the whole Neovim session and is not undone when switching to another configuration.

Other editors support this per run configuration (IntelliJ "Environment variables", VS Code launch.json "env").

Proposal: add an optional env field to setup_project configurations:

require("flutter-tools").setup_project({
  {
    name = "Dev",
    flavor = "dev",
    env = { API_KEY = "abc", LOG_LEVEL = "debug" },
  },
})

The values would be merged on top of Neovim's environment and only applied to the run/debug process of that configuration: the job runner (plenary Job env) and the debugger runner (launch config env). Other jobs like pub get or devices stay unchanged, which keeps the scope small (see #89).

Happy to open a PR if this sounds good.