extensions/configuration-editing/src/extensionsProposals.ts/provideInstalledExtensionProposals
export async function provideInstalledExtensionProposals(existing: string[], additionalText: string, range: vscode.Range, includeBuiltinExtensions: boolean): Promise<vscode.CompletionItem[] | vscode.CompletionList> {
	if (Array.isArray(existing)) {
		const extensions = includeBuiltinExtensions ? vscode.extensions.all : vscode.extensions.all.filter(e => !(e.id.startsWith('vscode.') || e.id === 'Microsoft.vscode-markdown'));
		const knownExtensionProposals = extensions.filter(e => existing.indexOf(e.id) === -1);
		if (knownExtensionProposals.length) {
			return knownExtensionProposals.map(e => {
				const item = new vscode.CompletionItem(e.id);
				const insertText = `"${e.id}"${additionalText}`;
				item.kind = vscode.CompletionItemKind.Value;
				item.insertText = insertText;
				item.range = range;
				item.filterText = insertText;
				return item;
			});
		} else {
			const example = new vscode.CompletionItem(localize('exampleExtension', "Example"));
			example.insertText = '"vscode.csharp"';
			example.kind = vscode.CompletionItemKind.Value;
			example.range = range;
			return [example];
		}
	}
	return [];
}