Add support for disabling nesting in the sinppets menu

[zadjii-msft]

Adds a nesting param to showSuggestions. The default remains "enabled". We now also accept "disabled", which will plop all the snippets into a single top-level list. This will make it easier for users to open suggestions based on what they've already typed at the commandline. e.g. type gitco at the commandline, and have that pre-filtered to the git commit -m ""\u001b[D snippet.

As spec'd in Suggestions-UI.md

---

It's dangerous to go alone, take this:

    "actions": 
    [
        {
            "command": 
            {
                "action": "showSuggestions",
                "nesting": "disabled",
                "source": "all",
                "useCommandline": true
            },
            "id": "User.showSuggestions.noNesting"
        },
    ],
    "keybindings": 
    [
        {
            "id": "User.showSuggestions.noNesting",
            "keys": "ctrl+shift+space"
        },

[DHowett]

Could we just make nesting "transparent" when you're searching? I don't think anyone would truly have enough categories that a well-targeted search should need to go through multiple levels.

[zadjii-msft]

Could we just make nesting "transparent" when you're searching? I don't think anyone would truly have enough categories that a well-targeted search should need to go through multiple levels.

[zadjii-msft]

Closing this one out for now. Just disabling nesting when there's search text does seem sane. BUT to do that, I need to:

• merge snippets pane (Add a snippets pane #17330)
• Yank FilteredTask into it's own file
• Do the "I can filter on the input as well as the name"
• then do this

because it doesn't make sense if you can't also include filtering the literal typed commandline.