Apply ability to select render for terminal

[AndrienkoAleksandr]

Describe the solution you'd like

It would be nice to apply ability to select render for terminal widget. It should be useful for qa, because currently we are using canvas render for terminal and qa can not read content from widget. QA could use dom renderer and read content from terminal widget without problems. VSCode has such feature, so we can apply it to the upstream theia to align with VScode:

VSCode has options: dom renderer and canvas(faster)

[rhopp]

This would be SUPER helpful! Didn't know, that something like this is possible!
As this has major impact on tests development, I'm setting this as P1 (feel free to disagree ;-))

[mmorhun]

Fixed in eclipse-theia/theia#6471