Add devtools.debugger.show-content-scripts to the prefs set by web-ext run command

[rpl]

In addition to document the new UI to show content script in the tab devtools toolbox (tracked by mozilla/extension-workshop#1960) we should also add it to the prefs that are implicilty set in the profile that web-ext run creates or pre-configures.

As a side note, we may also consider if we should remove the devtools.chrome.enabled pref from the one we are setting.