Proposal: Support object favorites

Object Favorites

Support user customization of variable presentation by allowing users to choose their "favorite" variables. Debug adapters can then return these variables as the top-most items in variables requests, as well as customize the displayed value of variables with "favorite" children.

VS Feature: https://devblogs.microsoft.com/visualstudio/pinnable-properties-debug-display-managed-objects-your-way/

interface Capabilities {
    // ...

    /** The debug adapter supports adding and removing favorites on variables. */
    supportsObjectFavorites?: boolean;
}

interface VariablePresentationHint {
  // ...

  /**
   * Set of attributes represented as an array of strings. Before introducing additional values, try to use the listed values.
   * Values: 
   * ...
   * 'canFavorite': Indicates the item can be added as a favorite of its parent type.
   * 'isFavorite': Indicates the item has been added as a favorite of its parent type.
   * 'hasFavorites': Indicates the item's current children contains at least one favorite item.
   * ...
   */
  attributes?: string[];

  // ...
}

/** 
 * Adds a variable to its parent variable's collection of favorites. 
 * Clients should only call this request if the capability 'supportsObjectFavorites' is true.
 */
interface AddFavoriteRequest extends Request {
    // command: 'addFavorite'
    arguments: AddFavoriteArguments;
}

/** Arguments for 'addFavorite' request */
interface AddFavoriteArguments{
  /**
   * The reference of the variable container for the variable to be added to its parent's favorites.
   */
  variablesReference: number;

  /**
   * The name of the variable in the container to be added to its parent's favorites.
   * Valid variables have a VariablePresentationHint attribute of 'canFavorite'.
   * Valid variables do not have a VariablePresentationHint attribute of 'isFavorite'.
   */
  name: string;
}

/** Response for 'addFavorite' request */
interface AddFavoriteResponse extends Response {
}

/** 
 * Removes a variable from its parent variable's collection of favorites. 
 * Clients should only call this request if the capability 'supportsObjectFavorites' is true.
 */
interface RemoveFavoriteRequest extends Request {
    // command: 'removeFavorite'
    arguments: RemoveFavoriteArguments;
}

/** Arguments for 'removeFavorite' request */
interface RemoveFavoriteArguments {
  /**
   * The reference of the variable container for the variable to be removed from its parent's favorites.
   */
  variablesReference: number;

  /**
   * The name of the variable in the container to be removed from its parent's favorites.
   * Valid variables have a VariablePresentationHint attribute of 'canFavorite'.
   * Valid variables have a VariablePresentationHint attribute of 'isFavorite'.
   */
  name: string;
}

/** Response for 'removeFavorite' request */
interface RemoveFavoriteResponse extends Response {
}

CC: @andrewcrawley, @weinand

Start with the proposed Capabilities, VariablePresentationHint, AddFavoriteRequest, and RemoveFavoriteRequest definitions in the issue. Review how these definitions fit the debug adapter protocol, then verify that favorite capability signaling, presentation attributes, and add/remove requests are specified consistently. Done means the object-favorites proposal is fully defined for clients and debug adapters.