Implement conditional visibility for custom views

[sandy081]

Ability to define a condition when a custom view can be visible or not

[sandy081]

Implemented as part of #27823

[sandy081]

To verify:

• Contribute a custom view using extension point views
• Implement and register a tree data provider to this contributed view using the API.

Or you can use our samples here - https://github.com/Microsoft/vscode-extension-samples/tree/master/tree-view-sample

Verify that views are shown / hidden based on the when context value you provide while contributing the view.

[weinand]

@sandy081 I verified with my Loaded Scripts Explorer and it works great!

"views": {
	"explorer": [
		{
			"id": "extension.node-debug.loadedScriptsExplorer",
			"name": "Loaded Scripts",
			"when": "inDebugMode && debugType == 'node'"
		}
	]
},

[orta]

If like me you're trying to figure out how to make your viewport only show up if when your extension has declared something loaded, the key is using the "setContext" command: vscode.commands.executeCommand("setContext", "is-running-storybooks-vscode", true) which you can then use inside a "when":

    "views": {
      "explorer": [
        {
          "id": "storybook",
          "name": "Storybook",
          "when": "is-running-storybooks-vscode"
        }
      ]
    }

( Figure it out by reading this example )