Automate screencast recordings

Created by: marionebl

This is a spinoff motivated by manual work done for #3810.

This PR automates said work completely but introduces a non-trivial ammount of additional scripting and depedencies.

• screencast.js: Automate screencast.sh, asciinema, svg-term-cli. Removes progress-bar, npm tree data from cast
• screencast.sh: Simulate user input, trigger demoed commands
• screencast-start.js: Start a shell command and end the process log patterns have been observed