Make jump to code a standard event

Within the Dart extension, we use a custom event to indicate to the IDE client that we would like to jump to a specific spot in code. Our current use case for this is allowing developers to run a mobile application and click on an element in the application to move to the code that corresponds to the UI element (see https://github.com/Dart-Code/Dart-Code/blob/master/src/extension/commands/debug.ts#L559 and https://github.com/Dart-Code/Dart-Code/blob/master/src/extension/commands/edit.ts#L21). We could also see this type of event being helpful for performance tooling where data could be associated with a point in code.

interface CodePointerEvent extends Event {
  event: 'codePointer';

  body: {
    /** The file requested. */
    uri: string;

    /** The line requested. */
    line: number;

    /** The column requested, optionally. */
    column?: number;
  };
}

Start by reading the current custom event usage in Dart-Code's src/extension/commands/debug.ts around line 559 and src/extension/commands/edit.ts around line 21. Compare those payloads with the proposed CodePointerEvent shape, then define the protocol change and verify that the documented event covers file, line, and optional column targets.