Bug 1819767 - [devtools] Expose ChromeUtils.isDevToolsOpened to know if...

Bug 1819767 - [devtools] Expose ChromeUtils.isDevToolsOpened to know if DevTools are debugging something in the current process. r=smaug,devtools-reviewers,jdescottes

This doesn't help know what particular resource DevTools is currently inspecting,
but at least it helps know if it debugs something:
* one or many BrowsingContext(s) for regular DevTools (you can use BrowsingContext.watchedByDevTools instead)
* the whole process for the Browser Console/Toolbox

Differential Revision: https://phabricator.services.mozilla.com/D173295