Update xterm.js

- Create onSpecificOptionChange and onMultipleOptionChange helpers xtermjs/xterm.js#4195
- Fix demo again xtermjs/xterm.js#4193
- Switch to Ubuntu 20.04 xtermjs/xterm.js#4192
- Fix clearTextureAtlas call as implemented on IRenderer xtermjs/xterm.js#4180
- Create theme service xtermjs/xterm.js#4188
- Ensure stale bitmap is not used when drawing new characters xtermjs/xterm.js#4189
- Lint rule for on=event emitter and rename all methods with on prefix to handle xtermjs/xterm.js#4187
- Fix a bunch of memory retention problems xtermjs/xterm.js#4185