feat: allow to restrict on which OSes actions allowed to run

[norskeld]

Could be useful to allow specifying OS requirements, both on single actions and suites. Allowed OS values will map to std::env::consts::OS + explicit "all" (which should be the default).

With single actions:

actions {
  run "command" os="all"
  run "command" os="windows"
  run "command" os="linux"
}

With suites:

actions {
  suite "init os="all" { ... }
  suite "win-specific" os="windows" { ... }
}

With actions in suites (overrides value defined in a suite node):

actions {
  suite "init" os="all" {
    run "command" os="macos"
    run "command" os="windows"
  }
}

[norskeld]

Closing in favor of #16.