This adds a "fish_greeting" function that prints the variable of the
same name.
In doing so, it makes $fish_greeting default to a global
variable (this is of little cost because of the `_` builtin)
This means that:
- We have fewer universal variables by default
- If we change the default greeting people will actually get
- it (unless they have a leftover universal, of course)
- If the user changes their language the variable changes with it
