QuickPickItem button API support

[TylerLeonhardt]

Example:

const quickpick = window.createQuickPick();
const item = {
	label: 'Accounts',
	// NEW
	buttons: [{
		iconPath: new ThemeIcon('accounts-view-bar-icon'),
		tooltip: 'Eyyy'
	},
	{
		iconPath: new ThemeIcon('gear'),
		tooltip: 'zxcv'
	},
	{
		iconPath: new ThemeIcon('callhierarchy-incoming')
	},
		QuickInputButtons.Back
	]
};
quickpick.items = [
	item,
	{
		label: 'HELLO',
		// NEW
		buttons: [{
			iconPath: new ThemeIcon('callstack-view-session'),
		},
		{
			iconPath: new ThemeIcon('debug-breakpoint'),
			tooltip: 'Eyyy'
		},
		{
			iconPath: new ThemeIcon('debug-breakpoint'),
			tooltip: 'zxcv'
		}]
	}];

// NEW
quickpick.onDidTriggerItemButton((e) => {
	window.showInformationMessage((e.item === item) + '');
	window.showInformationMessage((e.button === QuickInputButtons.Back) + '');
});

// The top buttons. Not new.
quickpick.buttons = [
	{
		iconPath: new ThemeIcon('accounts-view-bar-icon'),
		tooltip: 'Eyyy'
	},
	{
		iconPath: new ThemeIcon('gear'),
		tooltip: 'zxcv'
	},
	{
		iconPath: new ThemeIcon('callhierarchy-incoming')
	},
	QuickInputButtons.Back
];
quickpick.onDidTriggerButton((e) => {
	window.showInformationMessage('top button: ' + e.tooltip);
});
quickpick.show();

This PR fixes #88716

Is this testable? I feel like integration tests can't test this because it requires someone to click on a button.

[chrmarti]

For testing could the QuickPick object have a triggerItemButton function that is not in the API interface and triggers the code for handling button clicks in the renderer? (Maybe that function could only be added when running in 'test' mode? If that's easily detectable.)