src/vs/workbench/api/common/extHostTypes.ts/SnippetString/appendPlaceholder
class SnippetString:	appendPlaceholder(value: string | ((snippet: SnippetString) => any), number: number = this._tabstop++): SnippetString {

		if (typeof value === 'function') {
			const nested = new SnippetString();
			nested._tabstop = this._tabstop;
			value(nested);
			this._tabstop = nested._tabstop;
			value = nested.value;
		} else {
			value = SnippetString._escape(value);
		}

		this.value += '${';
		this.value += number;
		this.value += ':';
		this.value += value;
		this.value += '}';

		return this;
	}