Add viewContainer contribution point

[thegecko]

As discussed in #200880 we are keen to add a contribution point to the debug viewlet.

This proposal implements this as well as allowing contributions to other view containers.

To use this proposal, the package.json would look similar to:

{
        ...
	"enabledApiProposals": [
		"viewContainerContext"
	],
	"contributes": {
		"commands": [
			{
				"command": "proposed.dummyMenu",
				"title": "Dummy Menu",
				"icon": "$(beaker)"
			}
		],
		"menus": {
			"viewContainer/title": [
				{
					"command": "proposed.dummyMenu",
					"group": "navigation",
					"when": "viewContainer == workbench.view.debug"
				}
			]
		}
	},
        ...

An example can be found here: https://github.com/thegecko/vscode-proposed-debug

Partially fixes: #200880

[jrieken]

It this be viewContainer/... instead of view/container/..?

[thegecko]

It this be viewContainer/... instead of view/container/..?

I've renamed this to viewContainer/. The original format was based on items such as this:
https://github.com/thegecko/vscode/blob/9d4274e5599f6ba2c667a54ec0d1d6594d30145b/src/vs/workbench/services/actions/common/menusExtensionPoint.ts#L242

Readme and example updated to reflect the changes above.

[roblourens]

Discussed with @jrieken earlier, pushed a change to require that this contribution point only apply to the debug view container, for now. Also, the proposal is mentioning "context menus" but this is only about the title menu, right? I'm renaming the proposal, but otherwise this is ready for Insiders :)

addressed