Switch to new WebIDE scanner / runtime API (bug 1069552)

Makes use of the new scanner / runtime API in WebIDE.

After this change, the add-on will still work with pre-API Firefox.

I've also added a Firefox for Android support, as it was helpful to build it to try out the runtime API.
That command fails on my unagi as there is no `grep` :/
It ends up reporting a custom Fx on Android...
All other shell commands works, but this one.
Could you switch to `cat` and grep in JS?