Make registerMarker's cursor offset optional

[Tyriar]

We've been using markers more and more in xterm.js and found that every case so far looks like registerMarker(0)

We should make the argument optional and have it imply 0 (same line as the cursor).

xterm.js/typings/xterm.d.ts

Lines 860 to 866 in 6c18b7f

	/**
	* (EXPERIMENTAL) Adds a marker to the normal buffer and returns it. If the
	* alt buffer is active, undefined is returned.
	* @param cursorYOffset The y position offset of the marker from the cursor.
	* @returns The new marker or undefined.
	*/
	registerMarker(cursorYOffset: number): IMarker | undefined;

[Tyriar]

This is done

xterm.js/typings/xterm.d.ts

Lines 943 to 949 in 1c7c4eb

	/**
	* Adds a marker to the normal buffer and returns it. If the alt buffer is
	* active, undefined is returned.
	* @param cursorYOffset The y position offset of the marker from the cursor.
	* @returns The new marker or undefined.
	*/
	registerMarker(cursorYOffset?: number): IMarker | undefined;