src/vs/workbench/api/common/extHostTypes.ts/SnippetString/appendVariable
class SnippetString:	appendVariable(name: string, defaultValue?: string | ((snippet: SnippetString) => any)): SnippetString {

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

		} else if (typeof defaultValue === 'string') {
			defaultValue = defaultValue.replace(/\$|}/g, '\\$&');
		}

		this.value += '${';
		this.value += name;
		if (defaultValue) {
			this.value += ':';
			this.value += defaultValue;
		}
		this.value += '}';


		return this;
	}