Enable extensions to contribute tiles to the welcome page

[JacksonKearl]

Add a contribution point for getting started entries such that extensions can provide getting started content in their package.json.

[JacksonKearl]

Oops wrong ref, the above closes #116412

[JacksonKearl]

Entries look something like:

		"gettingStarted": [
			{
				"id": "ext.hello",
				"title": "Hello!",
				"description": "world",
				"button": {
					"command": "extension.helloWorld",
					"title": "Say Hello"  
				},
				"media": {
					"path": "hello.webp", 
					"altText": "Helloo" 
				}
			}
		],

[JacksonKearl]

This would be rendered like so:

[kieferrm]

@JacksonKearl which when clauses are supported?

[JacksonKearl]

Uses our standard IContextKeyService#contextMatchesRules service, so should be the same as any other place context keys are used.