fix: fix regression in xterm 4.15

Attaching a customKeyHandler prevented the typed character from being printed.

Close #59

Author: javier-godoy

src/main/resources/META-INF/frontend/fc-xterm/xterm-element.ts

@@ -97,6 +97,9 @@ class CustomKeyEventHandlerRegistry {
 		context.dispatchEvent(new CustomEvent('CustomKey', {detail: ev}));
 	}
 
+	//https://github.com/FlowingCode/XTermConsoleAddon/issues/59
+	let core = (context.terminal as any)._core as ITerminal;
+	(core as any)._keyDownSeen = false;
 	return handled;
   }