Editor should support export and import

[wenerme]

The grammar is easy to copy and paste, but the examples is also very important, consider allowed write examples in grammar or export import json

write examples in grammar like

/**
 * @example
 * ```
 * t: 1 + 1
 * f: 1 + a
 * ```
 */
Arithmetic {
}

https://github.com/harc/ohm/issues/396