Allow forced registration of extension by default

[buhrmann]

This is to avoid ValueError: [E090] Extension 'test' already exists on Doc, in case the Emoji class is instantiated more than once in the same process.

Incidentally, this is the case in the repo's tests. Although the language object is defined as a fixture with function scope, and each function therefore receives a new copy of the language object, the instance of the Doc class used for registering the extension is shared. The tests were hence failing when repeatedly trying to register the same extension. This should be fixed now with force_extension=True by default.