Update `currentBrowser` Core util and browser naming conventions

[haydnba]

currentBrowser method of core/src/support/Utils needs to be updated - currently, seems not to properly handle Safari browser feature detection adequately.

Minimally we should safely handle Chrome, Firefox, Safari, Opera, MS Edge.

Also, we should update the browser name conventions found in core/src/support/Browser enum with 'FIREFOX' instead of 'EXTENSIONS' in preparation for API wrapper updates....

Relates #146 etc.

[porkbrain]

The original motivation behind naming it EXTENSIONS was that FF adheres to a standard which seemed to be shared also with Microsoft Edge. Now ME has moved to chromium though.

[haydnba]

Yes, it seems now that Safari will reintroduce JavaScript based browser extensions that they are following closely the 'browser' namespace WebExtensions API used by Firefox (although it is equally possible to build the Safari extension using 'chrome' namespace as well, since they have created their own build tool). Meanwhile, Chrome, Edge (MS) and Opera are all using the 'chrome' namespace. So we have a situation now that half browsers use 'browser' and other half use 'chrome'....