keyboard: Make it configurable whether an action exits cursorless mode

[pokey]

It is surprising when a command exits cursorless mode. Let's turn it off by default, and support config like the following:

  "cursorless.experimental.keyboard.modal.keybindings.action": {
    "t": "setSelection",
    "b": {
      "id": "setSelectionBefore",
      "exitCursorlessMode": true,
    }
  },

Note how

• It mimics what we already support for VSCode command config
• It is actually defined on a per-keyboard-shortcut basis, so they could in principle have one version that exits and one that doesn't

Fwiw here is an older attempt to make list of commands that exit keyboard mode configurable #1912

@josharian does the format above match your expectations? Is id the right key?

[josharian]

Looks great.

[josharian]

Ended up using actionId as the key, as it read better in other contexts. Happy to adjust as desired, though; we can discuss on #2174 as needed.