Make it easier to bind keyoard shortcuts to custom REPL commands

[PEZ]

When creating a custom REPL command it is a bit of a roundabout way to bind a shortcut to it. If Calva predefined some easy to remember shortcuts to easy to remember custom command keys, it would be a matter of assigning one of those keys to a custom command definition to use it as a shortcut.

The current command for bringing down the custom commands menu is ctrl+alt+space. We could change that to be ctrl+alt+space space and then pre-bind ctrl+alt+space <whatever> to custom commands with key <whatever>.

<whatever> could be things like 0-9, a-z, enter, tab, right/left/up/down, and some more single character things found on most keyboards.

Example. With this custom snippet definition:

    "calva.customREPLCommandSnippets": [
        {
            "name": "Remount CLJS App",
            "key": "r",
            "repl": "cljs",
            "ns": "example.app",
            "snippet": "(start)"
        }
    ]

Typing ctrl+alt+space r would call this snippet.