Add text highlighting for TTS

[ztefanie]

Is your feature request related to a problem? Please describe.
As a user with low reading skills, I want to see which part the tts feature is currently reading out.

Describe the solution you'd like
Implement a text highlight, e.g. a light yellow background, of the text part that is currently read out with the tts feature.
Add this for web, android and ios.

Describe alternatives you've considered
Close this issue if it is too much effort.

-blocked by #2916 #2917 #2918