Add debug window context menu contribution points

[thegecko]

As discussed in #200880 we are keen to ensure there are contribution points in all debug window context menus

This proposal implements contribution points for the watch, breakpoints and disassembly views.

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

{
        ...
	"enabledApiProposals": [
		"debugContext"
	],
	"contributes": {
		"commands": [
			{
				"command": "proposed.dummyMenu",
				"title": "Dummy Menu"
			}
		],
		"menus": {
			"debug/disassembly/context": [
				{
					"command": "proposed.dummyMenu"
				}
			],
			"debug/watch/context": [
				{
					"command": "proposed.dummyMenu"
				}
			],
			"debug/breakpoints/context": [
				{
					"command": "proposed.dummyMenu"
				}
			]
		}
	},
        ...

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

Fixes: #200880, #150161

Relates with: #212500

[thegecko]

Due to the existing context menus already utilising referenced objects in their commands which cannot be serialised (e.g. due to circular references or bigint types), a mechanism is required to determine if the context menu item has been contributed by an external extension.

The method chosen here relies on a source existing for external contributions. I'm not a bug fan of this, is there a better way?