create real API for "debug.logToDebugConsole" command

[weinand]

Currently we support a 'debug.logToDebugConsole' command to send output to the debug console.

We should introduce real API for this.

[weinand]

I've added this proposed API:

	/**
	 * Represents the debug console.
	 */
	export interface DebugConsole {
		/**
		 * Append the given value to the debug console.
		 *
		 * @param value A string, falsy values will not be printed.
		 */
		append(value: string): void;

		/**
		 * Append the given value and a line feed character
		 * to the debug console.
		 *
		 * @param value A string, falsy values will be printed.
		 */
		appendLine(value: string): void;
	}

	export namespace debug {
		/**
		 * The [debug console](#DebugConsole) singleton.
		 */
		export let console: DebugConsole;
	}