support more flexible ValueFormat #197

Variables and Evaluate requests support an optional format field if the debug adapter can supportValueFormattingOptions. But the only option ValueFormat currently available is whether it's hex or not.

interface ValueFormat {
  /**
   * Display the value in hex.
   */
  hex?: boolean;
}

Our debugger backend (go delve) can offer more flexible formatting options (based on a go-specific formatting verbs users can provide, e.g., %v, %x, ... ) and I wish we could take advantage of it. What do you think about adding a free-form string 'format' field? I can imagine other language debug adapters may have their own, language-specific way of formatting their data types.

I looked into a way how java debugger support a variety of formatting options (custom command to switch the formatting + invalidated event), but I am afraid that disallows different formatting per value (I may be wrong here).

ps. Is VSCode utilizing value formatting options any where? I don't see a control knob that allows me to select hex format. I can file a separate issue in the other repo for that.