[change] Default profile preferences (exclude loading extensions from OS scope)

I propose to add
extensions.enabledScopes = 5
to default preferences for new profile.

Purpose is to isolate FirefoxPortable from running environment a bit and load only extensions form user profile or those associated with application.

In practice: this will restrict searching for exts to paths:
{FirefoxPortableDir}\App\Firefox\extensions
{FirefoxPortableDir}\Data\profile\extensions

while excluding exts listed by registry or located in appdata profiles for current and all users.

More on scopes (no anchor, search page for "Installation scopes" header)