Set line-height of xterm.js' text area to 0

It's taking up space when it has focus which breaks the terminal's position. This
only seems to happen on Mac for some reason.

Fixes #12038

src/vs/workbench/parts/terminal/electron-browser/media/xterm.css

@@ -42,6 +42,7 @@
 	width: 0;
 	height: 0;
 	z-index: -10;
+	line-height: 0;
 }
 
 .monaco-workbench .panel.integrated-terminal .xterm .xterm-helper-textarea:focus {