support multiple keybindings for some actions

[devoncarew]

• support multiple keybindings for some actions
• run is now cmd-r (as well as the older dartpad binding, cmd-enter); fix https://github.com/dart-lang/dart-pad/issues/2818
• format is now cmd-s (as well as the vs code binding, shift-option-f); fix https://github.com/dart-lang/dart-pad/issues/2848
• added support for the older dartpad binding for quick fixes - option-enter; address part of https://github.com/dart-lang/dart-pad/issues/2868

Note that the behavior of cmd-s changes with this PR; before, it runs the snippet, after, it formats it. cmd-r is probably better as a run keybinding (and will probably prevent people from unintentionally reloading the dartpad page), and cmd-s should be familiar to vs code users as many have format-on-save active.

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