Profiles: Support transient profiles

[joaomoreno]

We could introduce the concept of a transient profile which gets automatically deleted once the last window which uses it is closed.

Additionally, combined with #151301, we could have a CLI way to start VS Code on a transient profile.

cc @weinand

[weinand]

@sandy081
I've tried the --profile-transient CLI for extension debugging, but that had no effect.
But creating an empty profile "mock" and then using --profile=mock in the launch config works great.

(Yes, I've turned on the "profile preview" flag and I'm running latest out of source)

Here is the launch config that didn't work for me:

		{
			"name": "Extension",
			"type": "extensionHost",
			"request": "launch",
			"args": [
				"--profile-transient",
				"--extensionDevelopmentPath=${workspaceFolder}",
				"${workspaceFolder}/sampleWorkspace"
			],
			"outFiles": [
				"${workspaceFolder}/dist/**/*.js"
			],
			"preLaunchTask": "npm: watch"
		},

[weinand]

Using the --profile-transient flag in extension debugging works fine as long as a workspace folder is specified in args.
If no workspace folder is specified --profile-transient is silently ignored.