Toggle and focus VSCode's integrated terminal

Add these shortcut bindings to toggle and focus the terminal. Hitting `⌘+t` again will hide the terminal.

```json
{
  "key":     "cmd+t",
  "command": "workbench.action.terminal.toggleTerminal"
}
```

and

```json
{
  "key":     "cmd+t",
  "command": "workbench.action.terminal.focus",
  "when":    "!terminalFocus"
}
```

gabrielfortuna
April 6, 2021