According to spec:
- The Enter, Tab and Backspace keys will not have release events unless
"Report all keys as escape codes" is also set.
- Key events that result in text are reported as plain UTF-8 text, so
events are not supported for them, unless the application requests
"key report" mode.
I checked kitty's behavior. While it does report release events for keys
that result in text as CSI sequences, press and repeat events for these
are reported as plain UTF-8 text. This commit changes Xterm.js to match
this behavior.