src/vs/editor/contrib/snippet/snippetParser.ts/FormatString/toTextmateString
class FormatString:	toTextmateString(): string {
		let value = '${';
		value += this.index;
		if (this.shorthandName) {
			value += `:/${this.shorthandName}`;

		} else if (this.ifValue && this.elseValue) {
			value += `:?${this.ifValue}:${this.elseValue}`;
		} else if (this.ifValue) {
			value += `:+${this.ifValue}`;
		} else if (this.elseValue) {
			value += `:-${this.elseValue}`;
		}
		value += '}';
		return value;
	}