Support grouping in output events

[weinand]

A proposal for supporting "groups" in DAP output events:

A new optional enum property group is added to the Output event with these values:

• start: start a new group in expanded mode. Subsequent Output events are members of the group and should be shown indented. The output attribute becomes the name of the group and is not indented.
• startCollapsed: start a new group in collapsed mode. Subsequent Output events are members of the group and should be shown indented (as soon as the group is expanded). The output attribute becomes the name of the group and is not indented.
• end: end the current group and decreases the indentation of subsequent Output events. A non empty output attribute is shown as the unindented end of the group.

See microsoft/vscode#34981

[weinand]

Here is the TS version:

/** Event message for 'output' event type.
	The event indicates that the target has produced some output.
*/
export interface OutputEvent extends Event {
	// event: 'output';
	body: {
		// ...

		/** The output to report. */
		output: string;

		/** Support for keeping an output log organized by grouping related messages.
			'start': Start a new group in expanded mode. Subsequent output events are members of the group and should be shown indented. The 'output' attribute becomes the name of the group and is not indented.
			'startCollapsed': Start a new group in collapsed mode. Subsequent output events are members of the group and should be shown indented (as soon as the group is expanded). The 'output' attribute becomes the name of the group and is not indented.
			'end': End the current group and decreases the indentation of subsequent output events. A non empty 'output' attribute is shown as the unindented end of the group.
		*/
		group?: 'start' | 'startCollapsed' | 'end';

		// ...
	};
}